I wouldn't try to re-invent the wheel. Once you have isolated the tannin, you still have to come up with a formula to work with it in a developer. Photographer's formulary sells PMK pyro kits, or for that matter, pyrocat, both of which are superior developers.

Seems to me you would be spending more time in rendering the oak galls than in taking pctures and developing them. Why set up a lab when the finished product is in no danger of going away commercially? tim