I used to fly a military aeroplane which would operate above 50 000 ft with ease - I've done 45 deg AOB turns in it watching Phantoms falling out of the sky trying to catch it! We used to have a limit of 45 000 ft using the normal oxygen regulator as even 30mm overpressure was insufficient in the event of rapid decompression above that height if 'aerodynamic suck' reduced the flight deck pressure below the ambient static pressure. To go above 45 000 ft, it was necessary to wear a pressure jerkin and to have an improved regulator which could deliver a 50 mm overpressure. The PJ pushed against the body preventing physiological damage, but those who did the high altitude rapid decompression run in the chamber (25 000 ft to 55 000 ft in 3 seconds?) said it was a pretty unpleasant experience - and if you didn't wear the PJ then you were limited to 40 000 ft as the effect of 50mm without a PJ would be very dangerous!
To be certficiated to cruise at the heights intended, Beoing's Sonic Cruiser is going to need a similar approach to decompression protection as Concorde. Plus, no doubt, little cabin windows as well!!