currently there is no way to do this.
It would be cool though, i definately would use it. The only way i can see how this would work is to make another screen that only has that one button and that screen is forced to stay on top no matter what. So in essense it is possible, there are many pop up screens in RR that stay on top, but are usually only used for brief moments, like the gps menu, exit menus etc.
If you could tag a screen with an allways on top setting
allow to click through to the under screen
allow the loading of 2screens at the same time
then it would work.
You could do some cool stuff, like put buttons directly on the map (like skinbedder), or shape a window.
The more i think about this the cooler it sounds