You guys are funny, dry sump society....
David, two things control oil seep into the crankcase. The first is the height of the tank, if it is appreciably higher than the pump inlet, the height can exert some pressure(measured in inches of water, or in this case, oil) on the pump gears. The second is the closeness of fitment in the pump gears. You could put a solenoid in the line or a valve, but I wouldn't. Too much chance of forgetting the valve or failure of a solenoid valve.
I don't know about the CB pump, but I do know that the Autocraft and Bugpack pumps have suction gears that are about 2x the length of the pressure side. Just like the 911 pump.....so any oil that may be in the crankcase is gone in a second or two of startup.
It is easier to control the height of the tank in a Spyder, there is more room in front of the engine. I have zero oil migration into my old "wet sump", even after the winter of sitting. What is funny is my dipstick is still in the engine to cap the tube. But I still use it. Just not in the normal location. I modified and installed a Vanagon dipstick tube, it is much longer and made it easier to check the oil with the old system and the 911 shroud. It's about a foot long, so it allows me to use it in the dry sump tank, then store it in the engine. I know it's weird, but it works well.
FYI, there is no crying in the DSS. HA!