I think I can see where you’re coming from, but what you’re proposing isn’t the same as what has been proposed in the past. Having a mirror reveal more of the map is what has been proposed previously, and is fairly straightforward. Projecting the contents of the map “mirrored” is actually pretty complicated, and has to override the map rendering code.
Also the two effects interact badly. The one kind of mirror can reveal terrain around a corner, but stays consistent because it just shows what is actually there. The other kind of mirror displays a false image, but maintains consistency because it only projects this false image into an area that it blocks. If you combine the two you cause a situation where you can see around a corner with mirror type 1 into an area that mirror type 2 is using to project a false image.
Funsizeninja: do ?1 and scroll down to Peek, I don’ recall what it’s mapped to. When you peek, you can shift your point of view to a square next to the one you’re actually on, so shift around a corner to check for monsters without exposing yourself.