The database idea is way, way out there - that would take a bunch of MySQL researching. You would have to have a common symbolic link (or the right entries in my.cnf and my.ini) to the data files (that would be all you could have in common). It might work - MySQL really doesn't care (from an internals point of view) what OS it is on. I just don't know enough of the super-technicalities of it.
As far as the partition - it would probably need to be FAT32 (just a guess) as it supports the CIFS spec.
You won't be able to control anything that is in the serverspace that is on the shared partition that needs execute type access, I don't think. FAT32 doesn't support that, nor Unix style file permissions. Maybe you could do some .htaccess methods to get around that. Saying that, I think the Linux side will be the tricky thing -
My best guess is that it would work for static content, but is likely to be a headache for anything else.
BTW, XAMPP for Linux is nothing at all like XAMPP for Windows, other than the Sample apps and common elements in the 'XAMPP Security Concept' and the fact its an all-in-one installation. Its much more like a traditional LAMP stack, and you need a fair knowledge of Linux at the CLI level. (if you were thinking along those lines)
I'd also be very interested to see someone make this work. many, many, many challenges. I think if you search enough, someone did something close, I just don't recall the details.
Good Luck and keep us posted. Be sure the coffee/tea/beer/whatever is on hand and ready...