The QRH states that when landing configuration is established you should decelerate to your calculated approach speed in final approach. I think the best way to do this would be to use selected speed. Notwithstanding what you said about GS Mini, the approach speed calculated from the performance tables as a result of the failure wouldn't be calculated by the FMS anyway. So the only way to fly that speed in managed mode would be to insert it into the FMGC, which just seems unnecessary to me.
But, as always, I reserve the right to be wrong.
