You don't know what needs to be done to make ARR a "SteamOS" game. That much I know, since you even didn't use the search function in the forums.

ARR on PC is a DirectX9 game. It would need a port to OpenGL. That's no small feat, since there are implied plans of an upgrade to DX11 in the future, which furthers distance the game from a chance of being ported to OpenGL. There are also other issues, like input, sound, etc., but the usual champion of screwing ports of games is the graphical part. You see, ARR is so tied to Windows features that the launcher itself is a lousy Internet Explorer box for the login.

The chances are zero as of now. Unless they have planned this from months ago in development, there's no way ARR will present itself either on the initial line-up or in the first year of SteamOS.