The problem with this is that the webfinger spec allows additional parameters (e.g. "rel"), so if you do this you violate the spec.

(EDIT: Actually given it does a full regex match it might work; though it'll return supersets of the intended results if rel= constraints are preset, and possibly break if the rel= argument is put after the "resource=" in the URL so would need refinement; of course it depends on using a server which allows arbitrary regex based rewrites, which rules out a lot of object storage etc.)

It may happen to work for some clients some of the time, until it suddenly doesn't.