I decided to try and get in-game mission joining working. It took about 3 lines of changed code to make it work in Dogfight mode. Although, with the AI spontaneously dying in the missions it wasn't very useful unless someone left.
Does anyone know why Volition took this out - did they just not want to bother to try and add it for Co-op and TvT? Or were there some problems with it in Freespace 1?
This goes the same for Closed and Restricted gametypes - looking at the interface art, the hotspots PCX has the spaces for the Closed and Restricted buttons, but the spots for them in the actual image were removed.