I can't see any reason why the Schroeder frequency would be any sort of upper limit for directivity control. If anything, it's the very low frequencies where even the relatively large dimensions of the walls would become too small for pattern control.

It's a mute point though I suppose because most voice-range systems aren't corner loaded, as they are too far from the corner apex. It's the bass system, up to about 200Hz, that I'm referring to when I speak of corner loading.