Hacking sounds cool, but in reality it is bypassing and emulating. The IMA computer is not involved and no Honda code is being rewritten. The additional storage is completely separate from the IMA batteries and only connected in the same manner as the IMA motor/generator. By using lead acid cells, the need for complex control circuitry is eliminated.
Since the Insight requires about 10 HP to cruise, and the IMA motor puts out more than 10 HP, in theory the gas mileage could be doubled if the IMA ran at 50 percent.

That's it in a highly simplified form. The actual implementation is not trivial. Looks like Mike has a good handle on it.