How do I keep intentionally unmounted filesystems out of the GTK file dialog?

[Tom Horsley]

I notice on my rawhide partition, if I do something like Save As
from firefox and ask to browse places to save, the file browser
dialog I get contains all the labels for all the unmounted file
systems I intentionally left out of fstab because I didn't want
to screw them up while testing rawhide.

How the devil do I convince the annoying file browser there was
a reason those filesystems are not mounted?