I was wondering if the FCL provides a subpackage for 3D file format parsing, similar to what fcl-sound and fcl-image are? If not, I was hoping to code something like "fcl-3d" and possibly adapt some of the Assimp API for it. I would use its Delphi bindings, but Assimp isn't extendable post-compilation. I was hoping for units like, say, FPObjReader or something.
Not that I know, but there are 3d libraries writen in pascal that support various 3d formats one of them might help you on the top of my head there are 2 libraries
1) castle game engine which even provides an viewer you can try.
2)glscene which also has a viewer as an example for a number of formats.
Those might give you a good starting point.