A hydraulic cylinder connecting the systems would work. It maps voltage (pressure) between two disconnected systems through a ratio of windings (piston area).

A hydraulic cylinder connecting the systems would work. It maps voltage (pressure) between two disconnected systems through a ratio of windings (piston area). It only creates flow when there is a change in voltage (pressure) and current (flow), and if the circuit is open on the induced side then the transfer of energy is limited to the capacitance (compressibility) in the circuit.