Same problem here,,,"couldn't find weapon for weapon" error in Jedi Academy using the ultimateweaponsjk2 mod.
SOLUTION
The ultimate weapons pk3 was missing some items like wp_concussion, etc that WERE in the original game assets, possibly because it was made for Jedi Outcast? Dont know.
So....with 7zip open the original game assets1.pk3 file and copy weapons.dat file from the ext_data dir.
Then go into ultimateweaponsjk2.pk3 and paste it into the ext_data dir there. You will have to edit some- everything was identical except for some ammo values, so i changed them to the mod file values and voila, it works now.