grep the dmesg output to see what device it is assigned to, then mount it appropriately.
Code:
dmesg | grep -A 3 scsi[0-9]\ \:\ SCSI\ emulation\ for\ USB\ Mass\ Storage | tail -1 | cut -f 3 -d " " | cut -f 1 -d :
gives it to me on my home debian box... of course, you'd want toa adjust the grep for a different key or improve it to a regular expression to make sure you are getting the right thing to mount. Maybe.
So tell autofs to ignore it, and write a wrapper script to handle the mounting and just call the wrapper at the start of your backup script.