Distributed weapon mods, like in FNV is the best solution. Using the WMK resources should result in no problem producing weapon mods.
I'll even throw in that I'll make a WMX patch for TTW when all is done and ready.
As for making NPCs use weapon mods. That's....open for debate. It is technically possible to add weapon mods to NPC weapons, they just won't be visible on the gun when they have it equipped. Of course, doing this requires NVSE, and is actually a mod I have intention to make anyway. It would of course be cross compatible with any mod, which means it would work with TTW.