I've managed once to play games on android with an wiimote. It's possible on android and just use the bluetooth communication. Given that, I think it may work on OUYA right away. But I don't expect a not official controller (patented by nintendo) to be the only way for players to play.
All you need is the library, and then you ask users to push 1+2, and they will sync. Then you can use your library to convert the signals into strings, such as "A" and "B". Something like this library would do: http://forum.xda-developers.com/showthread.php?t=672884
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http://www.amazon.com/gp/product/B004SZU0T4/ref=as_li_ss_tl?ie=UTF8&tag=wwwcherokeele-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B004SZU0T4
Usability is a guess of course. :)