It is an old post, but an interesting matter. I don’t know much about udev, hal and dbus. but maybe at it is possible to check something in /sys. This link can help (inclusive the links on the page): HAL and device management
A problem seam to be at udev, hal and dbus are independant from the wm. So, the mount-umount operation as well as other actions (as to start an audio player or whatever) have to be done at the system level, not at the wm level. This: HOWTO D-BUS, HAL, KDE media:/ and this: HOWTO ivman can be useful.
This cannot add or remove something from the fvwm menu, but ivman can write or remove a file, maybe directly or with a seperate script, file that can be used by the menu generation feature of fvwm.
Another way would be to not use ivman but write a program, some kind of new fvwm module for that, but it will require more programmer skils.
I hope at this reflexion can help someone.