Hi
I dont use btrfs, but I do backup some qubes while they are running, by
mounting the private.img and private-snap volumes. I think this is what
you are proposing, and it will work fine. Mount the volume ro.
I use a disposable to take the backup of the qube, and spawn it for each
new volume. It works well, and I dont have issues.
private.img is the file that contains /rw for your qube.
The timestamped entries are backups of your private.img. You can revert
to these as you will.
I dont recognise private-dirty. Sorry.