I got a mod that lets me reskin all weapons to a different tier. It's purely for looks, of course- I've just always preferred conventional weaponry to energy stuff. The mod works fine with every weapon in the game, even the weapons I modded in. I can reskin the plasma rifle to the MA5B from Halo Reach, for example, with no problems. However, it doesn't work with any weapons obtained as loot. The usual option doesn't appear.