I tried asking for that, a particular moderator here disagreed pretty vehemently with me about it - but he has a tendency of doing that for most requests.
I mean ultimately a simple script (for someone with half decent knowledge) could do it, right? Like the rsync you mention, without the mount point?
Hell, I don't care if my jail needs to come offline, dismount the mount points, run the rsync backup, compress the rsync into 7z or rar, then restore mountpoints and reboot the jail once a week :/
One thing - I don't know how to ask this because I'm a newb but a jail is based on the existing umm code in the OS right? they are like a protected partition or something? So the jail, which boots a new instance of FreeBSD is infact based off the core files in the OS but can't break them somehow?
Would this complicate the process in making a backup, especially if you say, backup a jail from FreeNAS 9.2 and import it into FreeNAS 10? - is this perhaps why they aren't interested in such a feature?