Getting completely OT here, but mount points are the inverse of what you are suggesting. The ability to mount a folder as a drive letter has existed since at least MS-DOS v6.22, and likely much longer; I just don't have a DOS syntax guide from older versions around to check. The command used is subst and it works like a charm. It is still included in Vista today.

Mount points allow a drive to be seen as a subfolder of a different drive, rather than as a different letter. So instead of C: and D: for your boot and data drives you could have C: and C:\Data as the two "drives."