Chas came over tonight and we rode around this area for a while, trying SPDs for the first time. We set the spring tension WAY loose, and it basically worked just fine -- he had three or four "panic unbalancing" moments along the way, but was able to unclip and catch himself just fine every time. Did some fast crusing above 20 for a bit, THEN, we got caught in the rain, two minutes after we decided to head back to the house....
So we got pretty well soaked during the last ten minutes, but navigated the wet streets just fine without slipping and sliding. So now he's pretty confident with clipless!!
About the only hazard left to test is an unexpected dog attack....