From an ATC viewpoint, there is no problem with using runway beyond the upwind end piano-keys if there is lighting and markings allowing you so to do.
On runways with displaced thresholds, the threshold can be hundreds of metres from the actual runway end and only displaced for landing operations in the opposite direction. Take off from a runway with a displaced threshold often takes place well before the piano keys. For example 31L @ JFK the distance from start of runway to displaced threshold is approx 1000 metres allowing enough distance for many aircraft to be actually airborne before the threshold.
So, in summary, as long as there is no lit or painted runway end markings before the end of the runway, then the whole of the runway is available for landing from that runway's threshold to the end. The second set of piano key markings you pass on landing I believe are irrelevant.