You are right - currently, the marketplace integration only supports bindings and nothing else.
"Transport" bundles probably will never be allowed as a listing on their own right, because they do not have any end user relevance. Once Karaf features/archives are supported, you could define the transport bundle as a dependency to install together with the binding.
For now, my only suggestion would be to repackage the binding to also include the transport code within the same OSGi bundle; this will certainly make it harder to maintain/build, but it is the only solution that I see. But we can also try to review it with some priority, but I cannot promise anything (as I am currently enjoying some vacation ).