If you populate .hal-mtab when you mount, then hal will be able to unmount when the desktop user right clicks on a “drive” and unmounts it.
Timing is everything though, and if you populate the file at the wrong time, you can screw up hal. You’ll also have to worry about removing the lines that hal or that udev added if your udev rules can do the unmount.
One way around this would be to turn off hal completely and handle populating the desktop with an icon for the file system. You wouldn’t have all the features of hal, but it would let the user unmount the thing!
If you want it to work on older gnome versions for instance, the desktop icon is created differently. It won’t mean anything to the user if you use the wrong version because it won’t actually launch and do the unmount.