I suppose it is (still) not possible. You have to move data
  between a restored readonly volume and a (possibly newly created empty)
  read-write one.

  (to be clear, we are talking about two independent matters here,
  rvm migration, and volume dumps/restores)

Yes, but if dump/restore were truly dump/restore, like a FFS
filesystem, then this wouldn't be such an issue.  I would like to see
restore be able to restore into RW replicated volume.

If you move data from the restored volume into a rw volume (mv? tar?),
how do the acls get moved over?  Isn't keeping acls most of the whole
point of using codadump instead of tar.  I do backups with tar,
because I want to be able to get at bits without coda if my coda
server explodes.