Markmar, Excellent post
I definitely agree that November thru February would be optimal. The reason for all this was because I think that Waltz was probably up at the spot on the ridge, late in afternoon, doing a security sweep of the West approach areas for any signs of company, when he noticed the hole in Palomino. I don't think he was looking for the hole, I think he noticed it due being sun lit from behind. "I think it caught his attention". I'm pretty sure that it would not be the easiest to see normally from that distance, but could be seen if you knew what and where to look, but not readily seen casually.
Other than illuminating the hole in Palomino, without blinding the viewer from looking towards the sun, I don't think there is anymore to it. The further South the sun is setting, the less it would be shinning in your eyes when viewing Palomino from the spot on that ridge behind the ravine. I think the time of year and time of day was just right for Waltz to notice the hole.
The information about using the Arch in conjunction with night sky is interesting. You have been on a research quest for a long time, and it shows.
Thank you for sharing about about all of this.
Sincerely, Idahodutch