I think instead, there should be a system in which when you press m5 it activates the watch, then double tap v 16 times in quick succession to trigger the cloak sequence. It's like the normal invis watch, except you lose cloak for every time someone on your team calls "spy!" But also, the cloak only works when you're being shot at by a brass beast heavy. The brass beast heavy in question can still see you, but he's the only one. It's also a bit like the cloak and dagger, because when you move the cloak saps your gun ammo and makes your knife become 20% less accurate. I think this is a completely balanced idea which makes as much sense as your original one.