In the end I got the Shimano M424s and Specialised Cadets.  The M424s aren't much worse with trainers than my previous Wellgo plastic platforms, but @Trengot is spot on that the spring-loaded bit doesn't do much of any use.  I wouldn't fancy wearing smooth-soled shoes on them but hiking boots or trainers are fine.

The Cadets are quite nice on platform pedals as well.  To get them to fit cleanly with the M424s I needed to file down some of the grips on the pedals (I could have cut away at the shoe as someone else has done, but pedals are cheaper and easier to repair (drill out and tap in grubscrews for example) had I got it wrong.  As for the walkable-shoes sceptics, you can't feel the cleat at all; you can sometimes hear it hit the ground, perhaps on a bit of grit.  The sole is claimed to be stiffness index 4, which makes it comparable to hiking boots, they're no problem to wear all day, and even jog a mile or so on hard ground.