There's no doubt that being a vegetarian on a tour through Africa is going to be tough. Indeed, it's tough even if you go anywhere in continental Europe (to French chefs: true vegetarianism does not include fish). The strange thing is that most Africans tend to be vegetarian, purely because of the high cost of meat. As soon as you head out to a restaurant, they usually lack much in the way of truly vegetarian options. African waiters usually are unwilling or lack the lateral thinking ability to possibly ask the chef to prepare a meal without meat.

Most of the riders who started the tour as vegetarians have started eating meat again - some of them are mainly vegetarian but will eat meat when there is slim picking. There is one vegan on the tour who has remained solid throughough. I've managed to avoid meat but have had to adjust my diet to compensate - both for the lack of options and the heavy strain we're putting on our body. Regarding the latter, I don't feel as if we receive enough protein on the Tour - the kitchen goes to some effort to make sure we get some protein (beans or chickpeas most of the time, tofu a couple of times!) but occasionally the dish is entirely vegetable based. At lunch too, there is often no vegetarian option that includes protein while meat eaters usually get some form of meat. (Understandably it's hard to find meat substitute at a lot of places but lately when the supermarkets have been well stocked, I fear that us vegetarians have been forgotten.)

I've developed a few tips for helping anyone who intends on staying vegetarian for the duration of the tour.
1) Don't be fussy.
It's hard enough finding vegetarian food sometimes that you can't insist on strict separation from meat products. At one place in Sudan that served burgers - the eggs were cooked next to meat burgers and often on top of them. They were perfectly happy to serve fried egg burgers but you had to take what you could get.

2) Find your supplements.
In order to recover at all (your body starts burning muscle on the longer days on the bike - protein is a must) you'll need to supplement your meals with food that is protein heavy. Almonds/peanuts are quite good for this, as are eggs. True vegetarians would argue that eggs are cheating but you'll really suffer if you won't / can't eat these. Many of the TDA dinners include eggs, and most African restaurants that can cook vegetarian dishes will usually cook eggs.
Protein powder is quite useful and many of the racers use recovery drink. I've been using whey powder since Nairobi where it was available in the malls. I would recommend developing some scheme for stocking up with protein powder (at least if you're trying to race or be somewhat quick) or just buy it when you can (Nairobi onwards) and ration it carefully. Care packages which you could get sent to each major city (with mild difficulty) would be perfect for this - if you have the budget of course.

3) Eat well on rest days.
Rest days are when your body is trying to rebuild all the weary muscle tissue from the previous riding week - you need to fuel it appropriately. I've felt a lot better when I've had several good meals on rest days. Unfortunately this isn't always possible so you need to remember to keep food spare. In Sesriem in Namibia where we are now, the restaurant at the campsite doesn't offer anything vegetarian and is unwilling to cook anything vegetarian (with the exception of fries). Consequently I've had to make my own arrangements and luckily had some quick cook noodles to hand.

4) Bring a big dish.
At dinner every rider will be served before open kitchen is called and anyone can come and grab how ever much food they like. This means that non vegetarians are welcome to help themselves to the vegetarian option (which they usually do). If you don't get enough food on the first round, there is a not insignificant chance that you won't get any the second time round (depending on what the non-veg option is - i.e. fish). Bring a big plate and you'll get more than enough the first time around.

I'll go through the countries and just briefly summarise the food available on rest days when we have to feed ourselves.
Egypt-
Chicken is quite a popular meat dish but so are falafels and koshary (a strange spaghetti/rice/curry blend which is delicious). Vegetarians will have no problem here.

Sudan-
Falafels are still quite easily found. Eggs are quite popular too. In most restaurants though, you'll struggle to find a good vegetarian selection.

Ethiopia-
Nearly every restaurant will offer a 'fasting' version of food which is meat free. The bread (known as injera) and curry combination doesn't sit well in most peoples' stomachs though so you may find yourself resorting to Western food. Usually this will be pizza or pasta and they'll usually offer a fasting version of this. Egg dishes (sandwiches and omelettes) are available at most cafés (as well as superb fruit juices). Not an issue for vegetarians.

Kenya-
The Kenyan staple, ugali, is often served with beans and spinach at local joints. At more upmarket restaurants though, it's usually served with meat and you'll have to ask to have it without meat (literally in Swahili: bila choma). Meat in general is big in restaurants. Eggs are easily found, as are chapatis.

Tanzania-
Catering on the safari was acceptable although the vegetarian option was fairly plain compared to the meat option. There's an excellent cafe in Iringa (the Hasty Tasty Too) which offers a huge menu and lots of vegetarian dishes. It's far from the campsite the TDA uses but well worth the visit.

Malawi-
I ate well at the Western chain restaurants and the campsite. We didn't eat any local food so I can't compare.

Zambia-
The campsites in Zambia are very touristy and we spent most of our rest day at a mall - vegetarian food was easier to come by. On the cruise down the Zambezi river though, the vegetarian option was terrible - a plain pasta dish. There's an (apparently) excellent vegetarian restaurant in Livingstone - the waitress never brought my order though so I will reserve judgement. The local food is meat based (at least at the fairly touristy restaurant we visited) so vegetarians will have an easy time ordering the single option available.

Namibia-
In Windhoek, the pricier restaurants will have a good vegetarian selection although the focus is on game meat which is specially available here. At the more cost effective fast food chains, there are few vegetarian dishes on offer - they will usually be out of stock too.

South Africa-
We'll see but I'm hopeful.

As for cost, I'm usually offended by how much vegetarian dishes cost compared to meat dishes. We pay the same price for buffets and on one occasion, the TDA organised a dinner on a rest day where I was forced to pay $5 for a single fried egg. At Western restaurants the price difference is slight - vegetarian dishes usually cost just a little less. Sometimes though, restaurants will exhibit logic and a vegetarian meal will work out much cheaper.

Finally, these are all my own observations and I may probably have overlooked something. It is possible to survive without meat with some slight added pain.