I would like to prevent random spawning, but allow spawns that are part of structure generation (like the ceiling spiders in mineshafts) or from spawner blocks. This is not quite the same as just requiring a random spawn to be located in a structure. I think it will need a condition on what kind of spawn is happening [random nighttime, structure spawn, or spawner block].

I don't get this exactly? You can already do conditions based on structure

You can condition on particular structures, but if there's a way to condition on being in any structure as opposed to no structure, that wasn't apparent to me from the docs.

Conditioning on being spawned by a spawner block would also be a plus.