Your idea will work. Personally I would get a secondary valve stack used (usually negotiated to about a 100 bucks) off a old l_M and plug the master valve to closed position and 'Tee' supply into the supply line to the oem valve stack. Run return to sump line. locate the valve anywhere convienient. Now run three wires, one to the oem master, one to the auger swing in, one to the auger swing out, connect the appropriate hoses plug the rest and presto, two way power. (use an auger swing momentary switch wiring so as to fire master on oem every time the switch is activated, either position)