Barring some clever optimization, it doesn't look possible to support both modes at the same time. It should still be possible to switch between the two modes by flashing different firmware. Not very user-friendly, but possible.

Unfortunately I've run into a wall getting any further than error -17 with the HD20 stuff. Nothing I do changes the behavior, and I can't find any good ways to troubleshoot what's going wrong, so I'm basically stuck until/unless a new idea comes to light.