Bösendorfer makes some large grand pianos with 92 or 97 keys. The 92 key one has 4 extra bass keys. I don't know about the 97 key one. The extra bass keys are under a little wooden cover that you can flip up if you want to play them, but I think the real purpose of the extra strings is to vibrate sympathetically with the regular strings, giving some additional bass oomph. Maybe the long strings on the Alexander piano provide oomph enough while staying more in tune.

At the upper end it's perfectly possible to reach the end of the treble scale, so maybe the 97 key Bösendorfer has extra treble keys in addition to extra bass keys. I haven't seen one though.