I can't see a way to do this so I think this is a feature request. :)

I'd like to be able to specifically exclude spawners from the spawning rules and still be able to modify the mobs' stats (health, speed, etc.).

My use case is that I'm using a mod that scatters spawners throughout the world and I want to eliminate hostile mobs from spawning except from the spawners.

If I'm reading the wiki right in order for any of the rules to effect spawners you have to use the "Onjoin" command in your code. Otherwise spawners shouldn't be effected.

https://wiki.mcjty.eu/mods/index.php/InControl#Examples_for_spawn.json