Fellow worriers....The scavenge side of the pump should not dry out on prolonged standing, the little non return ball valve on the pick up pipe in the sump is designed to prevent this. If this ball valve does leak, and the scavenge side is dry, then there is nothing you can easily do, except take the oil cap off and observe the return, fingers crossed. Which I know you all do.
With a manual tap in the supply line, gravity will prime the pump, if has run dry, by the time you have prepared yourself for the big kick. Beezermacc is correct that a dry pump will have very little suck and the danger with all suck operated valves in the supply line is if there is not enough suck the valve will not open to allow oil to prime the pump. This is because the spring pressure closing the valve has to be overcome by the cohesive force of the oil being sucked. No oil, just air in the pipe below the valve means no cohesive force, valve stays shut.
So, I would choose a manual safety type tap rather than an automatic valve. Plus regular use and a drain plug in the sump plate for the time I forget to turn off the tap.
Swarfy.