Does the HRP work with old user made mods?
Yes. Just load the mod files after the HRP and it should work. Note that if the mod replaces art in the original game the highres art will override them. This can be solved by adding undef commands after the loading of the HRP def code. See the def language documentation over at JonoF’s site. In the case of total conversions using the HRP is pretty pointless as most if not all the gfx is new anyway.