Yesterday I've been tinkering with winboard_x, to let it play FRC on ICC with winboard engines. There already was some functionality in there to enter FRC castling with King-takes-Rook moves, but I did not want to change the Baron to work with that. So now it will work with O-O and O-O-O like specified in the winboard protocol. KxR may still work, but I haven't tested that.
I've sent the adapted files to Alessandro to include it in winboard_x, but I'll send them to anyone who's interested.
Richard.