I've been watching the animated Flash Gordon series from 1979 on DVD, and in the commentary they mention having the movie rights, which they sold to Dino DeLaurentis to fund the tv version. (Filmation was initially going to make a live action Flash tv-movie, but the script became too expensive to shoot that way.) Which means either Filmation was in the right place and right time or King Features was more interested in having something done with Flash post Star Wars...
In the making of book for the first Star Trek movie, Lucas was one of many people considered for the director's chair. This was slightly before SW was out, (still referred to as "The Star Wars" in a Paramount memo) and the book doesn't elaborate if Roddenberry and Co. actually talked with him.