Union filesystems in general have had a storied and troubled past in the Linux kernel's political landscape. Valerie Aurora (formerly Valerie Henson), a former redhat kernel developer who worked on Union filesystems tried quite valiantly (no pun intended!) to get the unionfs patchset merged and ultimately left redhat to pursue personal things.

Somewhat recently there was some traction with a version of her patches that looked somewhat close to being merged, but it might have also been dropped.

http://lwn.net/Articles/482779/