I've become obsessed with trying to nab Africa as the Central Powers (against the computer; haven't ever done PBEM). I know it's a side-show and kind of silly, but I like it. I've found a few things that help:
1) If you are *not* at war with the UK for the first 2-3 turns (no Schlieffen Plan), moving the Marine division and some fast/transport capacity light cruisers to Africa helps. Depending on how long you have to wait for the UK to enter, you can wait for a few of the unlocking divisions to be free. What I really try for here is to get a fast fleet with ~1500 power and a lot of transport capacity, carrying a few divisions from Germany. My thinking is I won't really miss this naval strength (I probably won't be taking on the UK Grand Fleet anytime soon), the leftovers are more than a match for the Russians, and I don't need much transport capacity at this stage in the Baltics.
2) Pick which side of Africa you want to try for: East or West. West Africa is tricky because the French have a sizable force in Dakar, but Lagos is definitely achievable. East Africa (Nairobi) is tough but doable. Same for Madagascar.
3) Consolidate everything! Essentially, what I've been doing is moving all Atlantic, Pacific, and Indian Ocean fleets to Africa to pick up the small garrisons in German West Africa; I've also been sending the Goeben and Breslau, as the battlecruiser packs quite a punch. I then pick a jumping off point depending on which side of the continent I want to start on. I've been using the big European fleet to pick up the bigger German African divisions (Vorbek and the guy in South-west Africa).
4) Launch a big ol' naval landing at either Nairobi or Lagos or Madagascar. Use the fleet to bombard. It'll take a few turns to wear down the defenders, but these are all achievable with a land force of about 300 power (the miscellaneous African garrisons, Vorbek and the German SW Africa guy, and whatever I scrape from Europe).
5) Your fleet sometimes catches Entente shipping around Africa. I've caught numerous transports this way. I'd make sure you have a decent sized fleet in case anything big is lurking out there: once I had the Japanese sail over and cause some trouble.
6) Once you're rested and re-stocked, go for the next place on whichever side of Africa you're working on. The Portuguese may be in by now.
That's about as far as I've gotten. Once I was able to nab South Africa when the British forces there tried to get German SW Africa and left it almost undefended.
One thing I want to try is sending some Ottoman forces into Eat Africa via the Red Sea/Indian Ocean. I'm not sure how they'd do, but they could at least help garrison the areas I capture. My ultimate goal is to try for India, but I think that'll be super tough.
Anyways, just wanted to share this. Love the game!