I have thought about the wrist-weapons, and weapon attachments to armour too. I have yet to convert anything though. There are some rules in SE (and RCR I think) for these things - but to be frank I've had enough with coming up with solutions for the more conventional weapons. I do agree though, there's a lot more needed.

What you mean by "modifications" I'm not sure, I gather that attachments might seems different, but I think it's a nice solution to the modification issue. If you're thinking of small mods only to for example damage, encumbrance and/or perhaps add a quality or adjusting range I think that could be done too, but should be harder, cost more or in some other way be balanced - that is if you're thinking of bypassing the Hardpoint system. If this is not the case, I guess one could add a "customisation" attachment where the player himherself can pick and choose some basic modifier and optional ones. This should cost more HPs than similar attachments I think (not necessarily, but perhaps taking all the HPs of the weapon/armour), but perhaps cost a lot less cash. Also one has to do it oneself, and require a hard, or perhaps the normal average, mechanics check, but with an added setback die or two.