He was mainly a composer of orchestral and choral music (and one opera) and a longtime member of the CCM faculty.
Was there any relationship between him and Courage? Were they friends?
I think OneBuck's suggestion is very likely. From what I've seen in interviews, it seems that the film & TV guys were all buddies, and they worked together on some things, "bullpen" style. With that term I'm thinking of comic books: someone is on a deadline on an inking assignment, and a bunch of his comics buddies come over to his apartment, they get beer and pizza and together they all ink all the pages. I suspect that the film & TV composers worked together like that occasionally: Goldsmith "writes" something and Courage "arranges" it, then Courage writes another thing and someone else arranges it, etc. It just seems like that kind of working arrangement was common: Steiner chipping in with cues for TMP, etc. One imagines a tough work environment, with very little money being paid, all the control in the hands of the producers, and composers not wanting to risk getting labeled as being difficult to work with, or being late, or anything like that. Sweat shop conditions, with a lot of collegiality among the laborers.
If that kind of working arrangement was usual, and Huston was friends with Courage, then it seems very possible that Huston contributed to some season 3 scores; maybe even a lot of material.
So: not sure you have to completely discard this idea you've held onto for so long. But it would depend their being a friendship between the men.