So I started poking. It turns out that several launchers have mod slots, for things like grips and underbarrel and whatnot. However, none of the vanilla mods actually work with launchers. Adding “launcher” to the mod’s install options seems to make them work.
Also noticed that the “rifle scope” fits on the rail slot, while “holographic sights” fit on the sight slot. This means you can use both at the same time. I suggest making the scopes fit on the sights slot instead.