Do the bottom shield the one time and while it's off cut an access hole for the drain plug. But frankly for something like an oil change I'm amazed that there would not be an access opening there already. What sort of car is it?

In any event even if you can reverse the valves the passages through an air pump are far too small to be able to pump oil or any liquids effectively. The pump will end up feeling like it's a stiff shock absorber. You want a pump that was designed for liquids in the first place.