Before anyone else comes along, I've just learned how ridiculously easy it is to create categories of the type of your custom component- it's as simple as changing the &extension= parameter of the url. I think I'll just insert an int into whatever tables need use reference a category and then create a menu link to the categories component from within the cpanel of my custom component.
It's not exactly ideal, because I won't be able to filter lists within my component by their category unless I add more category type fields replacing int filters, but I'm pretty happy right now.
I'd be even happier if you added support for com_categories though