Support multiple capacities by type

The webform module supports fine-grained option submission limits in this way, it's architecture might be worth looking at.

But 3.x has sufficient room for customisation via events that I'm not sure a generic solution for allowing a single registration type to have dynamic capacity makes sense. Wouldn't such most such use cases need custom logic anyway?

But I guess from this retitling of this issue @john.oltman is actually thinking to allow setting a different capacity per registration type, not just one overall capacity for the host entity. Which would indeed make a lot of sense.