You could just test KeyHit for keycode=0 to keycode=255.
Looking at the system.mod source, it seems that every time a key is hit, Blitz increments one of the slots in a 255-byte array. KeyHit just retrieves key_hit[keycode], so a check for all 255 keys would be very fast.
Edit: Looking at the MacOS .m code for system, it seems that a fixed lookup table to convert from virtual key codes to ASCII values - isn't this going to cause a problem for non NZ-layout keyboards?
Looking at the system.mod source, it seems that every time a key is hit, Blitz increments one of the slots in a 255-byte array. KeyHit just retrieves key_hit[keycode], so a check for all 255 keys would be very fast.
Edit: Looking at the MacOS .m code for system, it seems that a fixed lookup table to convert from virtual key codes to ASCII values - isn't this going to cause a problem for non NZ-layout keyboards?