I guess that depends on your definition of "positive." The only positive encounter I can envision is being ignored, and I don't know that we know how often that happens.
If a LEO sees me lawfully open carrying with no suspicious behavior, I would expect him to react the same as seeing me lawfully doing anything else. I should not be of any interest to him
The best that any "interactive" encounter can be is neutral. He shouldn't waste my time, but if he does, as long as he is friendly and respects my rights, I would call that neutral.
Anything other than a brief "Hi, how ya doin?" is negative in my opinion.
Even by that definition, there are a number of posts about LEOs seeing an open carrier and smiling/waving, etc without approaching or bothering the person. Those are positive, great to hear, and I certainly hope we continue to hear more of them.