I am not sure this can help, but I thought that maybe a thread listing various sources of 3D assets that could be used in a DFSG compliant project could be useful, thus this thread.
I do not know enough about 3D formats and conversion tools toward NIF, but I suppose as long as there is the source code, it can be "not that hard(tm)" to do?
Please feel free to maintain this list since I don't think I'll stay around a lot
- flare-game's resources. Here is an example for a minotaur.
- Open Dungons. Licences and models.
- Unvanquished. Their accepted licences.