The reason Aces High is compatible with freetrack is because freetrack reports your head position in the same way that trackir does. Basically, anything that is compatible with TrackIr is compatible with freetrack as well.
Hattrack probably uses its own language. Maybe there is some kind of translator online, but I don't know. Seems kind of dumb that they wouldn't use the same language as TrackIR but maybe it's a legal thing.. who knows.