Thank you, Garth! What an education this is, and I really appreciate the time and effort you've put into helping me understand this.
As for how much play, if I had to guess I'd say that when I grab my wheel at the tire a move it laterally to test for play there's maybe a 1-2mm shift. Maybe? I don't notice it when I'm riding. Then again, I'm not even sure what I would be noticing if the wheel were flopping around as I ride.
However, I have noticed of late that sometimes when I'm braking in a corner there's a little bit of a shriek from the rear rim. This is, in fact, what got me thinking about the play again after coming to terms with it for some time. Could be just an improperly adjusted brake pad, but I did just change them (salmon thinlines), set them up properly, and still a little noise while cornering. Somehow I'm imagining the angle of contact between the brake pad and the rim changes slightly while in a corner due to the play in the hub, and this is what's causing the noise. But, I'm certainly not knowledgeable enough for this to be anything but an hypothesis.
I'm looking forward to investigating the RC vs AC bearings. In the meantime, my take-away from your note is that as long as I have radial bearings I should expect some play in the hub when off the bike.
Thanks again!
John