The workflow for paving a Mac with an older version of OSX than it's currently running is this:
- From an external medium, run an OSX installation that is at least as new as the OSX currently running. An older installation won't start.
- Start Disk Tools and erase the Mac's hard disk. Use the Journalling file system.
- Boot with the older OSX installation media inserted and the Alt key pressed.
- Install the older OSX.
Frustrating when software doesn't let me Do Dangerous Stuff :)
Next time i'll just use dban. Let's see how that goes.