After a little research I found that you can split a mesh by material in many 3D modeling programs, including Blender. You can then export each material separately.
That means you would assemble in Assetforge, then use something like Blender afterwards to take Assetforge export and generate a model file for each material. I saw a few videos on this as well.
It may also be possible to generate a png file from the materials and have the model material assignments replaced with uv mapping coordinates. This one I've seen forum posts on, but haven't looked for a plug-in for doing it.
Good luck.