tunefs on a mounted file system?

According to the man page, tunefs should work on a mounted file system
if it's downgraded to read-only. But, it doesn't work:

vir7:/home/ivoras# mount -u -o ro /
vir7:/home/ivoras# mount
/dev/ad0s1a on / (ufs, local, read-only)
devfs on /dev (devfs, local)
vir7:/home/ivoras# tunefs -n enable /
tunefs: soft updates set
tunefs: /dev/ad0s1a: failed to write superblock

I've set kern.geom.debugflags to 16 but it still doesn't work.

I think this may have something to do with GEOM access modes:

vir7:/home/ivoras# geom disk list
Geom name: ad0
Providers:
1. Name: ad0
   Mediasize: 8589934592 (8.0G)
   Sectorsize: 512
   Mode: r2w1e3
   fwsectors: 63
   fwheads: 15

I don't know where do the one writer and three exclusive users come
from. Any ideas where to dig?