So say that I use LeoCAD to export some brick models to Blender. Would it be unethical to modify the models in various ways (tris to quads tool, subsurfacing, textures, rigging, etc.) and then publish them to the internet, assuming that I do not try to profit from them in any way?
Almost certainly no, but with some caveats:
The vast majority (and I would assume the entire subset used by LeoCAD) are covered under the Creative Commons Attribution License 2.0:
You are free:
Under the following conditions:
However there are parts in the library that are not covered by the LDraw Contributor Agreement (CA), and these are covered by the more restrictive NonCA agreement. You are expressly forbidden from sharing, or making derivative works of these parts (creating 2d renderings of the elements isn't considered a derivative work, but converting the parts to a different format is).
All part files should have a license comment in their header stating either:
You should check those parts you wish to adapt, and proceed accordingly.