Hello 
Did you have a look at these topics ?
It seems there is no straight forward way since RTT is on GPU side while GUI is on CPU side (and moving image between contexts is expensive)
If you goal is to have another view in a corner like in your screenshot, you can achieve this by using another camera, and play with the viewport param (to render the second camera in a smaller part of the canvas)
I would say it’s because the highlight stuff is a post-processed, but using a second camera like I said previously, you will handle this as well :
++
Tricotou