I've often wondered the same when viewing these long/high-speed wheelies. I wonder how many blown engines happen? Or maybe they just overfill it a quart or 2?
I know a few of the stunt shops make mods designed to help keep the oil neck in the fluid while riding out long wheelies. Not sure if they just relocate it, build some sort of a baffle, or install a secondary pickup... But a few of em are at least thinking about it.
I've seen engines run 30+ minutes with no oil whatsoever, so I have no doubt the stuntahs could get away with that kind of abuse for a while. It doesn't take a hell of a lot of power to ride out a 10'o-clock wheelie anyway.
I don't think I could bring myself to stunt something that isn't dry sump though. DRZ is one of my dream stunt bikes.