Say e.g. I want a

Population

asset for each US state, and I’d like to be able to materialize them separately.

sounds like you want partitions https://docs.dagster.io/concepts/partitions-schedules-sensors/partitions#partitioned-assets-and-jobs

Ooh, I thought that was just for temporal partitioning, but you can use any discriminator?

indeed, via something like static / dynamic partitions