I'm guessing that it's possible to limit the memory Windows used through the BCD store (where the boot manager looks for operating systems to start). If yes, this should be possible by creating two boot entries to the same OS but with different memory settings.
I'm not sure about the details, but try googling the bcdedit
command - it's not terribly hard to use once you grasp the concept behind it, but of course extreme care is advised, lest you make your system unbootable instead.
But I'm sure someone else will be able to help you further with this