Reading this post, shows the nicety of keeping standby crews on hand to allow for late arrivals of the "real" crew. At major hubs does AA do this regularly.
AFAIK, every airline of any decent size has "ready reserve" or some other similar type of system where flight attendants are at bases ready to be assigned trips as needed. Whether they are used for boarding is a different question, I've not heard of it before either, but there's certainly no legal reason this tactic couldn't be used. My guess is that it's not common because doing it takes FAs out of the ready reserve pool for a given period of time (once boarding has commenced, the FAs can't bail on the plane), and the point of the pool is to give a cushion of FAs to allow flights to go if someone calls in sick, gets a flat tyre, etc.–having that person doing other work could potentially cause a cancellation down the line if that ready reserve FA then isn't available for the flight to Whereversville.