These things will be obsolete as soon as SunBeam releases the debug console enabler, but I couldn't wait for that...
Looks like grenades, shields and class mods are the simplest to modify they don't have any slot requirements like grips or magazines, you can add any effect to any item and it will work.
You can for example add 2 MIRA effects to a grenade and it will split twice, giving you 9x the grenades.
As for class mods, each ability rank stat is its own item, so if you have a 10 slot mod, you can just make it +10 ability rank for one ability.
There are however, some requirements for artifacts, but only as far as the tooltip legibility is concerned. For example here I don't really want the elemental damage effect but if I don't add it then most of the other effects won't show up in the tool tip and it looks less cool.