Very interesting scenario.
With the anticipation build up from the summer scouting and the excitement of the moment, the heart and head aren't communicating real well. The first thing would be get in control of the situation. Mind over matter, so to speak.
This could be your only shot opportunity for the season.
800 yds straight down and he's a herd bull. Got some size on him. You're gonna need a helluva horse.;)
I've never passed on a an elk shot, lack of opportunity, but have passed some decent muleys due to circumstances.;)
I'd just get into a somewhat comfortable setting position. Put the rifle on the ol' knees and sit there and ponder. If it ever gets comfortable, then possibly take the shot. This may not be so good as the only time I tried it, it was on a moose at 70 yards. I was waiting for him to turn. My hand must have got cold or the excitement was as controlled as I tho't. The rifle fired and resulted in a hit high in the leg. The good part was that it turned him.
While you are in this contemplative state, there may be some decent ideas come to mind. That's is, if you are not the type that 'locks' in on the ways things should be and can't accept the differences of that particular shot opportunity. Which usually ends up in a clear miss..
In similar situations I've see the Harris Type bipod slip at the shot. I've seen them fold up when shooting at a high angle uphill shot.
You may wish to tie up a rear leg of old Dobbin, ya know, the ol' lariat from the saddle horn around the hock and back to the horn then pull and the leg comes up. Then tip the ol' boy over, up hill of course;). He'll be as steady as a rock when you make the shot over the saddle skirt.
Of course you'll want plenty of pics of this.
And your horse will most probably not go huntin' with you again.
Situations as you present are the reality of hunting.
If worse comes to worse, observe, enjoy, remember and ride on.