Afaik, it does not work.
Yes, Haste should indeed affect Feral attack speed. However, since it is on the weapon, I highly doubt it will work. Kinda like the way everything else works. "On hit procs" on a weapon won't work, but if they're part of a trinket or something, it will still work.
I'd say try it cause it's supposed to be rather cheap, but I really don't think it'll work.