we've all practiced circuit and landing with the ASI covered up, haven't we.
Actually no I never have and I wouldnt have thought this would be common practice on safety grounds. In my humble opinion, airspeed is critical when in landing configuration on approach for obvious reasons. I once had a vacuum failure on an approach and was fortunate enough to have a G430 which displayed groundspeed so I made a faster than normal approach to give myself a greater margin for error but it wasnt something I enjoyed.