iOS and Mac were done a few months ago but now Diamond Chess Online on Android can also connect to a custom choice for FICS server. Nightmare-chess.nl is the only one I've heard of so far that's currently up and running. I know H.G. Muller used to run one for engine tournaments. That is really the idea. If someone runs a custom FICS server for engine tournaments, I understand code is open source on web, then the non player users, operators spectators etc, can login via mobile on Diamond Chess for either Android or iOS.
Here is the login UI on Android

And this is it on Mac. iOS is similar.
http://www.diamond-chess.com