Since this would be training, and not actual racing, what would be the range of bikes that will work for the routes you have in mind?
I mean minimum-maximum. Would a KTM 950 be too big? A 225 Serow too small? A rally bike would be ideal, but some people will be renting bikes for the African rallies, or don't have their rally bike yet. The ability to use a non-optimum bike for the training might yield more participants, since bike rental isn't viable.
1997 Moto Guzzi 1100i Sport
2005 KTM 525 MXC (plated)
1960 Lotus 7 (hey, it's a 4 wheel motorcycle!)