I can't test it right now (my dev-system is @home),
but it looks like the frame that is on the display when I unmap the buffer stays on it.

Oh, that's what you meant. That's how it should be. The kernel always holds a copy of the frame buffer which is the one than was last blitted. You need to blit an empty frame before you unmap or then you can turn off the display after unmap.

In theory you should be able to turn off the display with:
ioctl( m_fd, _IOW( 'd', 1, int ), 0 );

Either it'll run in the wild compo or maybe I can slip it in as a linux entry ...

Crossed my mine to put a wild compo entry to Asm'2k with a empeg demo but that would have required some time to put something together. It would be nice to see some demos and intros on empeg as well. The screen limits a bit but you can still do lot of nice effects with it. I wish the demoscene would evolve back again what it was some years ago; atleast the Asm'2k demo compo was a positive surprise and definitely showed that there are still talented people willing to do non-commercial stuff.

Kim