In 1.19.2 can the ceiling aware heightmap be used to generate custom structures other than hot springs using datapacks?

Yes. It works just like any other placement modifier.

So if I was making a dungeon that I wanted to spawn in the nether, I would put the line "project_start_to_heightmap": "simplyhotsprings:ceiling_aware_heightmap" in my [namespace]/worldgen/structure/nether_dungeon.json?

Sorry, my mistake - it can't be used in custom structures, only custom features.
It can't be used with "project_start_to_heightmap" because that option only accepts a heightmap. "simplyhotsprings:ceiling_aware_heightmap" is a feature placement modifier, not a heightmap.

Understood, thank you.