the only way i can think of.
have an animation that will scale to 0.0001 the sideweapon bone (or backweapon), when a script detects that you have that weapon equipped the animation plays, once you unequip the weapon again, have a second animation that will then scale the bone back to 1.
In theory this would work (weapon would disappear as you slealth it) but others might have a better way.