Hmm...

I think I might have an idea, not a very good one but it's worth tossing out there.

Mabey make another part of the model, not attached to the hands or the gun, and attach a transparent texture on it so that it's in the right place? I've seen some custom laser sight crosshairs made like that so when its animated it moves with the gun in an effort to make it more realistic.

E: I just realised that difrent FOV's would cause a problem with this.(even though its only 70-90)