Figured it out. I made a PFS called /olv (for "olvidable") where I put downloaded ISOs, Freenet stores, and anything else that I don't care if I lose, but don't want expired like stuff in /tmp/. It's empty on the DragonFly box but has some ISOs on the Linux box. On the laptop disk, /home is a filesystem in an encrypted partition, and /crypt and /olv are PFSes of /home/. I ordered the filesystems in reverse order of the mountpoint name: /crypt, /home, /olv. But you can't unmount a filesystem if a PFS thereof is still mounted.
Pierre -- li fi'u vu'u fi'u fi'u du li pa