Oil might be resistant to such migration, but fuel won't be. Trace amounts of fuel will leak through the clearance between plunger and bore, however small that clearance may be.
I am not sure, but I suspect the old "jerk pumps" that had separate lubrication for the bottom end of the pump used a mechanical seal between the fuel zone and the lube zone, which the plunger has to pass through.
It's worth noting that the VW P-D injectors have the plunger pass through the fuel return passage (next to no pressure in it) before they pass into the main pressure chamber. Any fuel that leaks past the plunger due to the very high pressure in the pumping chamber leaks harmlessly back to the fuel return passage. This way there is next to no pressure differential across the part of the plunger that separates two different fluids (engine oil on the outside and fuel on the inside).
http://www.zamslube.com/pd_diagram.jpg - note that the groove around the plunger up at the top is connected to a passage that goes back to the fuel return passage between the two O-rings on the outside.
Yes, I know a common-rail pump isn't a P-D pump. But it's a piston pump (and one that operates in the same pressure range) and the same concept could be applied ... but it would require an entirely different layout for the pump.