I don't know, to be honest. I think I remember seeing a metric fuckton of aiCreateSomeStructure() functions in the includes, but I'm unable to find them now. You can simply use new and new[] on all Assimp structures, but of course you'd need to link statically or export the necessary symbols to make it work.

Personally I think dynamic linking is stupid except for some rare conditions, so I never had these problems.