Measuring the no load voltage is not enough to test regulation, you also need to measure the full load voltage.
True enough. I was only thinking of overvoltage and didn't think to come up with a load for it. Maybe next weekend.
Recent model Pis don't care which direction the 5V on it's power rail comes from. They are perfectly happy with it coming from the micro USB power port, one of the USB downstream ports or the GPIO header.
That was what I was wondering.
The question is do you have a cheap and cheerful hub that violates the USB spec and feeds power back to it's host connector (and hence can power the Pi that way) or do you have a compliant USB hub.
This old Belkin probably predates the standards. I couldn't find any data at all on it and I don't have a stub USB-B end to test it with. Once my shiny new RPi shows up it'll be easy enough to plug it in and see. I chose this one because (1) I had it and (2) it has a clear case like what I plan for the RPi and (3) looked like a more-than-adequate power supply, with a clearly-visible power FET to regulate it. It also has a switch labeled self--bus on it that should serve as an on-off switch for the system.
Thanks for the prompt and informative response!