IIRC, the Fishman Fluence pickups the backplate was designed for, have internal preamps, so the 9v power runs directly to the pickups (Fishman Fluence pickups are very unique ... nothing else like them on the market ... no wire, no windings, instead they have layers of plates). I would work from an Alembic schematic and wire the backplate the same as the 9v battery. Should work fine.
Bill. tgo