I can comment on this, at least partially.
It is possible to import the models with 3DS Max, thanks to a plugin, however it was written for a specific version of the DFF file format, and a specific version of Max.
Being that Max is expensive, and the plugin is hit or miss anyway, that's not looking like a terribly viable option, even though it has so far yielded results for other people.
3D Object Converter appears to be able to import the DFF files, but exporting is something for the paid version ($35 USD), and I can't verify that UVmapping for textures or any rigging data for the skeleton is preserved with this option.
Noesis is a 3D model viewer/converter that has a DFF raw plugin, but does not seem to work with these files specifically. However, as it is free and the plugin architecture is relatively simple, I will be focusing my further research into a Battle Rondo and Diorama Studio plugin for the models.
There is also some documentation on the format, though a little vague and sparse, that was written by GTA3 modders (a game which also uses renderware DFF models). Not all of it appears accurate for Busou Shinki's DFF format version, but some of it does apply.
While I can't promise an exceptional amount, I can at least say that "importing these files is far from impossible" and I can go further to say that with the proper version of the proper tools, "it looks quite readily possible."
I can't say which version of Max and the DFF plugin for it are required for good results, as my 2013 trial version failed to load them, but the thread I read which included a picture of a fully loaded head, seems to suggest version 2010-2012... I will attempt to contact fatduck(the user that imported the files previously on another forum) for more information...
the .bin files for Battle Rondo and Diorama Studio are large archives and can be dumped using a script for QuickBMS. I'm not certain on the exact protocol for availing such information on this forum, as per the rules, just yet but if I find it's ok, I'll share that with you so at least one step of the process will have progress.
Much thanks to Sonja for this upload. I had actually registered hoping I might be able to get them for this very reason, so I suppose it all worked out rather well thanks to him.