I wish I had answers for you since I am planing on making my way there as well (this Winter). If you can read French, there are several informative discussions on the topic over on VoyageForum.com.
My feeling after all I've read is that a carnet is required for Ghana, but, as always, you can somehow get by with the help of a "facilitator" at the border. This thread
is good : one guy paid 15 euros for the service and another 50$.
say that a carnet (CPD) is not required and that instead, you can get by with an ATA Carnet
(issued for merchandise transport). This thread
on bourlingueurs.com gives the impression that the ATA carnet will work in some situations, but that it generally will not
(from another thread).
Good luck and do let us know what happens (I plan on buying a bike in Dakar at mid October).