I had a chance this afternoon to stop by my local university's library to flip through bound issues of National Geographic. Specifically, I looked at January-June issues from 1960, '63, '66, '69, '72, '75, '80, '85, '86 (I was thinking Halley's Comet might have spurred Questar to run ads), '90, and 2000.
Unfortunately, the person who bound the 1960, '63, and '66 issues did not include covers or ads that would have appeared at the front or back of each issue, so I can't say for certain if there were any Questar ads that would have appeared during those years. But the issues including and after 1969 that I looked at did have their full content intact. I didn't see any Questar ads.
The closest thing I saw were Meade ETX ads in the issues from 2000.
It's possible that Questar may have ran ads in National Geographic for a brief span of time that I did not touch on in my spot checking. But most of the advertising I saw in that publication for the years I browsed were from big-time brands (think airlines, automobile manufacturers like GM or Toyota, major camera producers like Canon, or appliance manufacturers). My gut feelings was that Questar would have been too much of a niche brand for National Geographic, and I have a feeling, too, that ad space would have been too expensive for a small company like Questar.