Anyone familiar with Ten No Koe 2? Want to mod it

Ten No Koe 2 plugs into the back of PC Engine and Core Grafx (like TurboBooster Plus without A/V ports) However it only uses batteries to preserve SRAM. I would like to change that so it'd run off PCE via expansion connector to charge a large capacitor (something like 5v 1 farad), and have the cap preserve SRAM when the system is off.

I couldn't find any schematic on this. I opened it and took a look, it's just a SRAM chip, resistors, transistors, diode, and nothing else. No propriety chip like the one in TurboBooster Plus and CD system.

 

Corection, there is another chip a Hudson HuC 6201 on the bottom. Probably to manage memory chip from the ext port.