A friend had this problem in one of his tanks. We came up with a novel solution.
For the past few years we have been frying turkeys for thanksgiving. The setup involves a propane tank, a burner, a huge pot, and peanut oil. The bird is submerged, cooks in about 40 minutes if I recall correctly, and is really moist and tasty.
We took his driftwood, and put it in the turkey fryer with water instead of peanut oil, and boiled the hell out of it. It completely fixed all his tannin problems with the piece of driftwood he had. The pot for frying turkeys is so big you can put a large piece or several pieces in at once.
Maybe you know someone with a big boiler setup, they are also used for crawfish boils. Or maybe just a big pot on the stove. Just boil it, and boil it some more. It'll take care of it.