The way to make concealable weapons work would be holsters as a clothing item, that way to cover them up you'd need to actually be wearing clothing :D
Obviously, only certain guns could be concealed. Drawing them should have a distinct animation and be slower than drawing from an open holster.
Another easy fix for this prior to the clothing idea would be the model only changes if you have weapons in your hot bar. If you don't have them in your hot bar you appear unarmed. This would give naked people the ability to conceal a weapon at the disadvantage of the time it would take to drag the gun to the hot bar and fire it if things get hectic.
However the best option for concealing would be to make it only possible if the person had clothing on. AKA at least full cloth
Could even have it so different types of clothing would make different types of guns concealable, for example an ankle holster wouldn't work with cloth pants, where a long coat would let you hide a sawn-off.
Also think inventory size should be decided by carrying a backpack or the type of clothing you're wearing, nakeds might only have a couple of slots, cloth would give you a few pockets, simple cloth bag gives you more room etc all the way up to coats/pants with pockets and proper backpacks. Not entirely realistic but at least you would be able to tell the real nakeds from the ones on a gathering run.
Edited:
Definitely a good idea, but surely if someone's going to kill you, won't they be more inclined to do so if they can see that you're definitely unarmed? This is why concealable weapons would be good, perhaps a hint at them being there on the body somewhere along with an equip-time penalty or something?
Only the people that want to KOS everyone would still do this. The people that kill for protection purposes wouldn't(AKA naked as a pipe shot gun and shoots you in the back of the head when your are not looking). I would say it would reduce the KOS nakeds by at least 30% assuming they implemented it properly.