It would be great if we could tell amanda that all subdirectories of a
specific root were to be treated as individual disk list entries. i.e.
balance all sub directories across backups.

This would help in cases like my home system where I have a few user
directories.
These are mostly static in size (grow slowly), on occasion I add another
home directory, for a guest or friend; updating the disk lists each time
is a pain and I often forget, so stuff gets left out of the backup.
These directories hold most of my data so splitting them up for amanda to
balance across backups is very useful.
As I want them all treated the same one entry referring to them all would
be great.