Cannot mount / from UFS labels

Hello,

I'm not sure this ever worked for 7.x but now I need to have the same
root fs device on two machines: labels to the rescue! As I don't want to
use the GEOM labels, but UFS labels, this is what I did:

# tunefs -L root / (in single user)
then updated /etc/fstab and rebooted

The kernel does not find this filesystem, pressing ? shows only
- a bunch of ufsid/* devices
- lots of partition names
- ntfs/System and msdosfs/HIDDEN which are other partitions on this
machine.

Only ufs/ comes up empty. What's my error?

Regards Uli