The Charm doesn't make it possible for lost-caste animas to exist.
If the castes/animas still "exist" in the metaphysics of your world, then the Charm would allow access to them. I'd say you need to know the caste pretty well- like, a First Age Lunar, or a Solar with strong memories of the relevant-caste First Age Lunar mate- and also know one other Exalt Ways.
You can learn Abyssal and Alchemical Exalt Ways because the core insight of the Charm is generic. It's a theoretical picture of Essence flows for Exalted animas. Once you learn something about the structure of Exaltation from that, elders can make new Exalt Ways (if they understand the relevant Exalted and caste, anyway) and students can move on to Games of Divinity Form.
One thing that strikes me about this Charm is that, if you interpret it the way Rebecca says, then you should, with deep enough study, be able to postulate new anima powers, given your insight into the power source and the way that Exalted sparks unfold out of them.
That's not really canon, because I don't think you can just look up from the table and say, "Hey, Storyteller, I'm buying a Chosen of Oramus Ways." However, I agree that if you make it cool for your ST, putting the effort and focus into it, you should be able to find valid but nonexistent animas.
Such ideas are 'stunt canonical' or 'deuterocanonical'.