If can find a SPDT vacuum solenoid switch that can vent the common port when electrical power is applied to the coil and connect vacuum to the common with no power, you could connect that to the IP shut-off valve. Vehicles in the 1980-90's used many vacuum solenoids for emissions when they had kludged carburetors. At least I have seen many Chryslers like that when in the junkyard.

Your plan to cut-off the lift pump inlet sounds possible. The suggestion that it could damage the IP by letting it run in air sounds silly. You are not letting in air, just reducing the inlet pressure. Similarly, you might bypass the mechanical lift pump and use an electric pump. There are many on ebay. I recall one for $35 says for gas or diesel, and I suspect all gas ones would survive fine in diesel.