Yeah, I asked the guys over there about it, around the same time I made this post. Radius claims it is something that cannot be done because the way Retroarch handles virtual inputs etc. etc. etc.... I'm calling BS.
At this point, I have pretty much just given up, and will be building a dedicated emulation box in the near future to connect directly to the TV. Kind of a shame as I was looking for an all-in-one solution. Overall, the input lag from Gamestream is just too high to play retro games comfortably. I am not one of those hyper sensitive input lag people (I don't think), but it can get pretty bad at times.