The Mosh license has been compatible with the App Store since October 2012. Nobody has done the port yet (which will probably require grafting on a GUI terminal emulator), but I'm hoping somebody will do it if the demand is really there.
https://github.com/keithw/mosh/issues/198#issuecomment-74179...
Panic has explicitly stated in the past that they won't add mosh support to Prompt due to license issues, for instance.