YOU CAN enter Nigeria without a Carnet. Same for Gabon , Namibia , Angola , etc..
the only "hard" country is Ghanna , althought people get in without a Carnnet from time to time .
Senegal is definitely doable without major hassles (bribes are around 100Euros to get in).
Definitely the trip is doable without a Carnet. In any case , perhaps you would be interested on register your own bike in Africa , as the rules concerning CDP applies on ly to euro registered vehicles.
With a motorcycle on African plates , you can simply sail trhought borders with absolutely no hassles/troubles at all.
I registered mine in Gambia for as little as 50 Euros (including the new , hand drawn number plates) , and has been the best decision on my trip.