Hmm. On both my plan9port and on a 9front system I find printarenas.c, but no script. Maybe you are thinking of the script for backup of individual arenas to file? Yes, that could be a starting point.
Anyway, printarenas.c doesn't look too scary, basically a loop checking all (or matching) arenas. It seems possible to modify the logic to start at a specific offset.
Not running fossil at the moment, btw., my main file server is a Linux box, but I use vac for backup, both at home and at work. Fossil is definitely on my todo list, although the reported behavior when running out of space is a bit scary. Do you know why it does not simply block further requests while checkpointing to venti, or even better, starts a snapshot before it runs out of space?