I've actually just got that working this morning. I've made a patch with the changes required for multiboot (against 0.2.10 rather than 0.2.5, so may need some adjustments). It adds a new ITEM_SCRIPT parameter for the .item files, and stops the multiboot script from unmounting all the filesystems before running the script. You also need to add a symlink for /bin/evkey in /usr/sbin (or edit the BackupMenu scripts to change the location). You can then add a multiboot entry with a ITEM_SCRIPT line pointing to the BackupMenu script.