GPS based landing systems use "differential" methods where a fixed receiver on the ground monitors its GPS derived position, compares it with its known position and broadcasts corrections to the GPS position. The inbound aircraft receives the corrections, applies them to its GPS derived position to obtain its correct position relative to the runway.
So, in theory, even if Selective Availability was switched on again it would not effect GPS based landing systems as the ground station would detect the GPS shift and broadcast corrections.
This is, I believe, the basis if the FAA's LAAS which has not yet been implemented.