Mike has the ultimate answer to some questions I have pondered for a while - as per usual! Thanks Mike.

Well there is some undocumented methods to make time machine work on non-locally attached storage, and although it might be disabled in patches - this works pretty well: As a regular user and in a terminal

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

Then when you visit the time machine preferences you can choose a network share as a TM location. Now it's very tempting to get a little mac mini for some zfs/nfs goodness [From using time machine on network shares]

This should let you use TIme Machine backups on any drive!!!!

Update

This really works - which is good news, but it does now always "take" unless you wait quite some time between keying in the command and opening the Time Machine preference pane.