The credit goes to Gilles for this answer; Gilles noted in the question comments that the '-n' switch ignores the mtab and unmounts anything listed in /proc/mounts.
From the manpage:
-n Unmount without writing in /etc/mtab.
So to answer my question of how to unravel a --rbind mount, this is the full command that worked for me:
grep /mnt/chroot/sys /proc/mounts | cut -f2 -d" " | sort -r | xargs umount -n
Merci, Gilles!