Got this working, eventually. This is maybe a little more superstition than established fact, but it seems that:
a) There must be a box collider present (even if collision is turned off for it) to act as the “volume” for the PCG node,
b) There must be a PCGWorldActor existing somewhere in the world. (I got mine by adding a PCGVolume to the map and then deleting it – the editor wouldn’t let me delete the PCGWorldActor that it added with the PCGVolume). I suspect this requirement could be met by deriving the C++ class that the blueprint is based on from APCGWorldActor, but didn’t actually try it, and
C) The generator must be set to “Generate at Runtime” – “Generate on Demand” doesn’t work.
I was also never able to get it to hit/not hit surfaces based on a tag, which might be a bug or just me doing something wrong.