The critical point is not how likely it would be that someone would choose to develop such an attack, but that
This type of Windows malware would be able to read and write the offline OS X partition completely bypassing all the OS X security, because OS X would not be running at the time.
Then, of course, there's the equally misplaced overconfidence of linux users
Again, how likely it is isn't the issue ... It's what it says about the almost religious fanaticism of some that leads them to glibly deny the possibility as an article of faith that's important - I knew I was right to give up on multiboot systems and go down the separate installations in drive-caddies route