I've successfully used a machine in the early days of W7 with '7 on one hard drive and XP on another. Switched in the 'order of boot' in BIOS when I needed to / felt like it.
These days, now I've proved to myself that W7 does all it says on the tin I've discarded the XP.
The authentication process counts the number of components which have been swapped (used to be 4 IIRC) before it concludes you are using a new machine, so, yes, your idea should work with no trouble at all.