Okay. Here's my post-purchase update.

I got sent out on a business trip right after I ordered my pedals and shoes, so I just got back yesterday and threw them on my bike. I set the tension as low as it goes, to make them easy to get out of, until I get used to the locking mechanism, and then I tested them with my regular shoes (a pair of Vans skateboarding-style shoes with a honeycomb pattern on the sole) and with the new clipless shoes (Shimano M038s).

The clipless mechanism on the PD-M647s is Shimano's "popup" design, so that it's easy to engage the cleat but it flattens somewhat when you put your foot on it. The mechanism is still too high for comfortable use with my regular shoes. The slick metal is pretty slippery. I think I'll be able to use it without risking injury, but I think the reversible pedals (like the PD-M324s) would be better if using regular shoes is really a requirement.

I was in my flip-flops (soft soled) when I first put the pedals on, and they seemed to flex around the mechanism well enough to make firm contact with the cage, so maybe a pair of softer-soled shoes would work better than mine.

That said, the PD-M647s are a nice pedal, though not so well suited to my original purpose. I'm pretty happy with the M038s, and I think I'll probably be using them more often than I expected to.