I'm still unclear on what you think is missing. I can't think of an application that will write its data to a random file system that may or may not be there (i.e. a usb key). If you are willing to have to always have the usb key mounted for Fritzing to work, then the hard link method should do what you want just fine. As would copying appdata and roaming (it needs both very likely) to the usb key manually (although that isn't as automatic). If the usb isn't present Fritzing will likely hang or complain about not being able to write files but that will be true of any application in a similar situation. I should also point out I'm just a random new user not a developer 
. The application itself is indeed static as far as I can see (modulo parts update, but it does that on startup) so there shouldn't be any problem with having appdata and roaming on a USB key that I can see, mine happens to be a different partition on the system disk but a USB key should do almost as well (it will be a lot slower than my Sata drive though at writing files).
Peter Van Epp