I just took a look at it - it probably -won't-, at least not by copying it into /etc/multiboot.d/.
You can -probably- get it to work the same by:
1. at the boot prompt, press C for console.
2. type "/path/to/BackupMenu.item"
If that doesn't work, make sure its marked as executable by running:
"chmod +x /path/to/BackupMenu.item".

Now, someone well versed in writing item files for MultiBoot -might- be able to convince it to drop to a shell and then excecute it, but I can't do it.

Either way, go ahead and experiment - Though the latter is fairly unlikely.

-Rob