I believe
@Wiimer mentioned this function was implemented in one of the beta FWs at some point, do I remember that correctly? If so, strange that it still didn't make it to the official release.
It would be really great to have the option to just measure the current response of individual channels directly from WHA, to be able to visualize the effects of both RoomFit and EQ. As you said, it can be done with REW but it is significantly less convenient.