If it is possible to detect something within a structure, and you can use the structure parameters in spawn, then you should be able to use it in spawner as well to allow for spawns exclusively in specific structures. Like how you can make mobs spawn in specific biomes, the same should be done for structures.

This will allow for the user to create custom spawn events that only occur within a specific structure as a condition parameter

This has been implemented a while ago using the new 'and' keyword that you can use in spawner.json. Sorry for not closing this sooner