I have never used Thinking Sphinx or even plain Sphinx, still, here are my 2¢:
It seems to me that your problem is that you have a meta-model toolkit on the one hand, your Category, Product, AttributeType, and Attribute classes. While on the other hand, you want to define concrete facets such as author, brand, etc. on top of that. That may be possible, but probably it is not.
I've had a quick look at the code for Thinking Sphinx and you'll probably be able to find what you need if you start studying it closely starting from its ActiveRecord integration. I don't think you will be able to achieve your goal without understanding details of how Thinking Sphinx works, as you're trying to stretch it beyond its intended use.