As far as my personal horses, I see the issue of personal space much the same way as you, Barry, with the certain difference of not being nearly as able to put it into words as eloquently as you do.
If I know the horse well, I don't guard my space any more than the horse tends to invade (by invasion I mean pushier, dominant behavior, not requests to get close). Scout is my buddy, he permits me to do what I will with him and "his" stall and pasture. All "touchy" buttons are gone, and I frequently clean and do barn chores around him, and groom him while he is loose in the stall, calmly munching his hay. If he gets nosy, being curious, I'll indulge him. Scout's barn "brother" is pushier, more dominant, and as I am not "his human" per se, I do keep track of him a bit more than I do Scout, but rarely do I ever need to defend my space, and even more rarely does he get defensive about his.
Unfamiliar horses are another matter. If I don't know the horse well, I keep one eye on his ears and the other on his feet. Until we get to know each other, unless I invite him to me or I go to him, if I can touch him, he's too close. Until we have a mutual understanding and respect for each other (I for his size, instincts, and quirks, he for my authority, trustworthiness, and ability to provide for his needs), he has no personal space, it has all become mine.
And he never really gets it back, since even if I allow him to put himself close, he does not dictate that I will yield to his space. It becomes a sharing sort of proposition, I think.
Really an interesting topic, I'm curious to see how others reply.