This may be very thing you wanted to avoid, but:
I've used IFOedit to set the 16:9 flags on the menu after the fact. That way, the playback device stretches it to fill the screen, and all the button overlays and everything stays in place. Of course, the image appears stretched, and the hardware stretching is uneven and artifacty (in my experience).
Guide here: http://forum.digital-digest.com/showthread.php?t=59409