It would be useful to have in device selection menu,
in addition to whole hard drive and partitions (hda, hda1..hdn),
a "MBR-only" item (e.g. to access just the first 512 bytes of the device).
Cloning a single partition it's usually enough (and it's the typical use)
but when the source partition has been resized, transferring the whole drive
could be a waste of time.
Of course, I'm referring to cast-o-matic generated images: with a
full-blown environment it's easy to "dd" and cast the MBR.
Another task that is pretty common in a classroom or lab environment,
is cloning the CMOS memory: I'm using a DOS based utility but
with a bunch of lines of code it should be possible to patch busybox
to handle CMOS memory operations.