Apologies, my mistake, it does offer other mapped drives, it just excludes letters used by non-network drives. (I could swear it was different when I tried earlier, but I must have tried the wrong thing. It works as you describe now, either way.)
Still, the underlying problem is (as you suggested) most likely that Windows isn't sending a notification about the change since both Opus and Explorer are affected.
Interestingly, if you fully exit Opus and then re-launch it, you get the updated label, so perhaps the Windows API caches network drive labels for the duration of the process or similar, unless they are disconnected before being changed.