Holding the power button in for about ten seconds is probably your best shot. There are a few other reset procedures in posts in the Toshiba HDD swap thread, but they all work via the remote, and if the unit is hung on wait, it probably won't respond to the remote. But heck, try them too.
The only XS I've run across that got stuck on wait didn't respond to anything except holding the power button in. The power led blinked and the fan restarted, but it still hung on wait. You could try leaving both the HDD and burner data and power cables unplugged and see if you get any kind of error. If you don't, it's likely a bad pc board, corrupted firmware, etc.
You did unplug everything (network, video, audio, etc.) from the XS54, right?