I think it _can_ be solved at the MacFUSE level using a transparent "stacking" FUSE filesystem that will cache appledobule files _locally_
It would be a very significant contribution to the Mac and FUSE communities if such a thing were built, as then all network FUSE filesystems like S3, FTP and SSH would see functionality and performance gains
Google can and has suspended accounts for the sort of excess activity GmailFS generates. Consequently GmailFS should not be considered reliable for backup purposes. Furthermore, the "Program Policies" prohibit the user to "Reformat or frame any portion of the web pages that are part of the Gmail Service" which is arguably violated by use of GmailFS, which, after all, "reformats" documents transferred via HTTP ("web pages") into data accessible via FUSE.