Write only usb stick

[ST]

Hi,

I need to implement "write only usb storage" in Gnome, i.e. users should
not be able access any external stuff through their usb
sticks/disks/cdroms but they should be able to save their locally
performed work on their usb sticks.
I thought to create a group "mounters" whose users should have the
privilege to mount external devices. And then let all users (through
sudo) run nautilus-sendto process as users of the "mounters" group. Do
you think its a correct approach? How can I implement it using
polkit(Gnome somehow automounts devices even for users that do not
belong to plugdev/floppy/disks/etc. groups)? 

Thank you very much!