One utility you may want to track down is called System Picker. I'm not sure off the top of my head who developed it or what its license was (freeware, shareware, proprietary, etc), but I do know it was included with an external LaCie drive I got many years ago. It allows one to switch between active systems on the same partition (think of it as the Startup Device cdev, but for selecting systems instead of drives).

I'll dig out my LaCie disks this weekend to find out more about this one.