A small thinking exercise.

Considering the system presented on our 4-strokes is basically a splash lubrication system, either with external oil tank (as nicelly shown and demonstrated here)or traditional fuel mixture, and considering most of the 4-stroke engines manufactured today use sealed front bearings, one could fill up the crankcase to a certain level with oil and by closing/removing the pressure nipple, oil would circulate throughout the engine parts, being forced back and forth by the pressure changes caused by the rising and falling piston. A wet sump system, if you like.
The backplate could be closed and used as an extra oil reservoir, only compensating for level changes due to blow-by and other small loses. Now that I think of it, it'd be roughly similar system to the one used on Honda's GX series[sm=idea.gif]
As a minus, as far as I can see, oil would tend to accumulate inside the rocker cover, and the valve stems/guides wouldn't be as well lubricated as they are by the fuel mixture we use traditionally during the intake and exhaust strokes.