Well thinking about it from a GRUB POV there are options but not one that is a solve all. I mean there are ways to be able to access the Linux dirve/partition but again you come across the same issue with that as you would trying to get Windows to boot from Linux.
I have read about explore2fs and while that seems like it could be of use it doesnt support Vista or Win7.
So while in theory what you are thinking could be accomplished it would be a lot of work.
Any way this is looked at it will take a lot of work and additional software to be installed for such action to be taken. It will require either explore2fs to be installed in XP to use iReboot properly if GRUB is the loader or run the risk of having your system bricked from having to access and modify the registry from Linux.
Lots of thoughts but sadly until there is a new boot method to come along that can access all file systems and do what is needed i dont think it will work.
There you go Guru. Your next project. iBootloader.
