[Addition to my own mail]

> If you want to experiment,
> dd if=/dev/?d0a of=/dev/?d0d count=16
> should make your system boot. (I didn't try)

Sorry, I forgot a
WARNING:
This overwrites the disklabel!
It must be reinstalled after that, or use
dd if=/dev/?d0a of=/dev/?d0d count=1
dd if=/dev/?d0a of=/dev/?d0d skip=2 count=14
instead.

best regards
Matthias