Okay. I think I've come up with a workable ruling. Tshern, feel free to comment.
1. You may consume a number of "points" worth of class features equal to: Your Final HD (70) minus your ECL at the time you gained the ability to consume class features.
2. You may not consume Spellcasting, Manifesting, Initiating, Soul Binding, Invoking, Essential Binding, or any like class feature. You may also not consume Wild Shaping.
3. Each class feature is worth a number of points equal to the class level it is gained at, except PrC class features, which are worth an additional 10 points above normal. (i.e. a level 1 base class feature costs 1 point, a level 1 PrC class feature costs 11).
Thus, if you gained the ability to consume class features at level 15, you would have 55 "points" worth of class features to choose among.
Thoughts? My other idea was to allow Spellcasting etc. but each level of casting/manifesting/etc. would cost two or more points. So it might cost 4 points to get 2nd level spells, etc.