Although not part of Network Rail's open data feeds, First Class info is now available via the public Live Departure Boards Web Service (LDBWS) API. Specifically as part of the coach data, which includes the class, loading level, number/letter and toilet status. There's one rather large caveat to this however in that LDBWS has the capability of showing it but very few services have it populated at the moment! Even so, integrating with the LDBWS API is not a simple matter, especially if trying to correlate it to other open data sources. Although it is technically possible, it's something I wouldn't expect to see on RTT. The best way to go about this currently is probably to subscribe to RailMiles.