It sounded very promising, but unfortunately nobody was able to come up with an implementation avoiding security holes. Linus put it this way, later in the same thread:
"As others have pointed out, there is no way in HELL we can do this
securely without major other incursions.
In particular, both flink() and funlink() require that you do all the
same permission checks that a real link() or unlink() would do. And as
some of them are done on the _source_ of the file, that implies that
they have to be done at open() time."