I don't see any reason you couldn't run a 'dual-boot' setup. (others may know better)
I currently run Tiger/FCStudio 1 on my internal MBPro system drive, then have an external enclosure with a clone of that drive on one partition and Snow Leopard FCStudio 3 on another partition.
IIRC, (and my experience has been) that booting from an external system drive must be connected via FW or USB.
Clone your current system drive, then create a new startup drive with two instances of OS. One with your current OS/FCP and the other with new OS/FCP-X.
If you're on a tower, I'd run these on different drives.
Can't speculate on DVDSP/BluRay.
K
EDIT - not sure if installing FCP-X would 'erase' FCP 7 or not.