Pneumatic tool oilers are designed to go on the high pressure side of the system and use a venturi tube to suck oil into the air hose. I doubt that would work on the pump inlet.
I'm not sure what would work well other than to make it really easy to put a few drops of oil in the inlet just prior to running the pump.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln