We got our Cameroonian registered motorbikes through Tanzania & Kenya (Both need Carnes) with an official looking piece of paper written in French that was actually a laissez passe for Cameroon. We got all the customs officials from the countries that don't need Carnes to stamp it saying that we needed to return the paper to Cameroon to prove we hadn't sold the bike. Then when we got to Tanzania & Kenya where they demanded a Carne, we presented them with our homemade documents which by then had lots of official stamps on it. They made a bit of a fuss but we pointed out that all the other countries (We had South Africa, Namibia, Zambia & Malawi)had accepted it so surely they should - and they did.
I'll scan in the document and post it on here.