I think that multidisc can already be handled in current svn: you put them in a single <software> (because they are such), and you put different discs in different "parts" (the list equivalent of ROM_REGIONs). with e.g. tags "disc1" & "disc2", see fm7_flop for examples.
then you can launch the disc you want with
mess segacd -cdrom game:disc1
for this it might be even more of help to have some sort of quikstart option (if you need specific disks to be mounted first), but I think there are more urgent aspects which need to be taken care of (e.g. multidisc handling in UI, given that the you can only mount the first disc from the UI at the moment and there is no equivalent of the commandline above)
Well my UI preference would be a 'related media' menu in the TAB menu (and maybe some simple previous/next buttons which could be mapped, that could get complex when you have multiple devices tho)
There are multiple ways the XML side of things could be handled, as I mentioned, it will require proper planning to ensure it works in the required cases :-) (allowing the user to add blank save disks too etc.)
Anyway, as you've said, plenty to work on for now anyway, the existing software lists still need bashing into shape in some places :-)