Would this be possible? It would be very useful for mobfarms.

Since the warp plate itself is a container, hoppers will always prefer to pick from its contents rather than any floating items above. I cannot change that, as the only way would be to block hopper access to the container, and others may be using hoppers specifically to pull or insert attuned shards into the warp plate.

You would have to use other means of item collection when trying to pick up items teleported through a warp plate.