Technically, I'd say nothing. But in today's market, who wants it apart from the few weirdos that hang out here? 😀 99% of the market is perfectly comfortable with other forms of focus confirmation.
I actually think there might be a little difficulty in projecting an RF patch. For one, because the patch would be generated from the sensor, it would be offset from the viewfinder both vertically and horizontally. A normal bright window projects a patch that is offset only horizontally, which, I presume, would at a minimum cause it to behave entirely differently.
For two, the patch would also be offset in terms of depth, because the sensor is not at the same distance as the VF. Further differences abound that way, and the difference would likely manifest itself differently at different subject distances.
Also, the patch would be generated from a sensor that would not be receiving an in-focus image unless it was already in focus, meaning the patch would not be effective unless you were already in focus.
And finally, the hybrid viewfinder system, if its rough schematics are to be believed, prevents the addition of a traditional bright window patch.
I considered all of these things not to refute you or anyone else, but because for the first week of this camera's announcement, I thought it offered this functionality. When I realized it didn't, I wanted to see why not, and... well, it seems pretty tough to implement.
A possible (expensive and unnecessary, ludicrously so) option would be to put a small sensor behind a window where a bright window would be, and to project THAT image, assuming it could be made to be sharp enough at all times, positioned at the proper depth, and offset from the VF only horizontally. But I wouldn't expect nor ask for such a luxury!
