I'd love a config option for cages that allows for tamed mobs to enter a cage regardless of the cage_catchable tag, but theoretically it would still adhere to the capture_blacklist tag, just for more control.

Why not just add stuff to the tag directly?

For a scenario like Ice and Fire's dragons. I'd only want people to cage a tamed dragon rather than an untamed one.

That's already a thing. Cages won't allow untamed tameable animals no matter what