I read your post very early on, but refrained from comment previously. But here goes, because of my experiences while hunting, of having other hunters scope animals very near me, and scoping me directly I will respond. In many cases when the animal(s) being scoped were close to me, it appeared that I was being scoped. Frankly those experiences are a bit unnerving. I mean, do I try to pull my trigger before they shoot me, sit back and let them shoot me... I had one guy in Utah years ago, clearly scoping me, despite me waiving my arms around to try to get him to stop. Honest to God, I shouldered my rifle and took the safety off as I pointed my rifle at him. So coming from this mindset, I just cannot imagine using my scope as a spotting scope. I simply do not point my rifle and look through the scope, at anything, unless I have already determined I am going to shoot it given a good shot opportunity. This comes from a guy that has been shot at, has had bullet fragments removed from his upper left shoulder, and has shot more than one person, killing them. I do not take the pointing of my weapon in any direction lightly, as the results can be un-reversible in the blink of an eye. By the way, that hunter in Utah was less that a split second away from being shot; the last thing I wanted to do. But I would rather be left dealing with the shit-storm that would have happened, and still be alive, then be shot by him. So despite your lack of seeing it as a safety issue, it has great potential of being a safety issue. But then, I have several times, pulled out the spotter to look at slight movement I spotted glassing at distance with my binoculars, only to find another hunter(s). If I did that with my rifle scope, it would be a safety issue (we do not need hunter orange in Ca).
I get your weight savings logic. But for the life of me, I do not see a 40 to 50 mm objective lens of a rifle scope gathering nearly as much light as a 60+ mm objective of a spotting scope, given the same quality glass, let alone the clarity of the larger objective with equivalent glass. Now if you have a cheap spotter but top tier glass in your scope, yes the top tier glass in the scope is likely to out perform the cheap spotting scope, in some cases by leaps and bounds.
I can also tell you from personal experience that looking at a mule deer buck a mile away through my ED 82 spotter, I can easily tell if it is a trophy class animal, or not. At a mile plus, I can see small kickers, small eye guards, and EVERYTHING any bigger with exceptional depth perception. high powered scopes (which I have from mid range quality glass to top tier) simply do not compare.
So with all due respect, and no knocking you (the OP) for your ingenuity in trying to problem solve (kudos for it actually), I just do not see this working out for you, for the above 2 reasons. But the greatest reason being safety, I mean lets say you do see an animal and glass it with your scope, but there is an unseen hunter near by that thinks you are scoping him or her; I just pray that it all works out well for all involved. Ultimately, this all comes down to individual choice. And, clearly, what you propose is legal. But since you brought up the safety issue in your post above, I thought my examples here might assist your choice in this matter.
Best wishes!