That would be pretty cool. So far I am pretty sure nothing. The program would have to convert from IOS to Python, to my understanding. Also it would require more components. An easy way would be with IP adress, but again, that would take a bit of programming skill as well. Good idea, though.
But if you really want a controller, go for a SNES usb controller. I am currently working on my Retro Pie and Still waiting for parts. To have fun, you pretty much need Snes USB, power supply , USB hub if you want, and A custom case( My parents have a big 3d printer, that I can use if I pay) Then you would have a PiePhone 5s XD ! Joking but SNES USB is really the way to go. My idea would probably have some lag in it.