But... for an Uno you want more than 5V for Vin, correct? "Recommended 7-12V" and "Limits 6-12V" is the spec on arduino.cc.
Not really. The 7-12v spec is so that the on-board linear regulator can regulate it back down to 5v (and not very efficiently either).
If you have a good regulated 5v source, you can bypass the Uno's regulator and feed 5v directly to the 5v pin. That is exactly what happens when you plug it into a USB port.
Aha, excellent to know. I was going by the advice on the ruggeduino site (you cannot power a ruggeduino from the 5V pin) and the warning on the Uno page at arduino.cc (5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.). These warnings are probably to keep newbies like me from letting the Magic Smoke out.
I think the key is as you say - a "good regulated 5V source" - and so the buck converter would do the job.
I shall buy one and experiment with it.