The underlying issue is that gravity won't be denied, simple as that. If only the solution were so easy. Continuous oil flow is needed to support this kind of riding, without compromising engine life. What is required is a sump that's always at the lowest point of the crankcase. That'd take two return lines, feeding a deep sump. I reckon that one return line could be plumbed into the oil drain hole, using the existing threads that now hold the drain plug. The other would have to be drilled & tapped, somewhere near the kickstart shaft. They'd need to be sizable lines, to deliver sufficient drainback rates. This might require 1" ID tubing. Keep in mind that the stock setup has close to a square foot of open drainback area and it all gets funneled (more or less) to the tiny sump, where the oil pump pickup is nearly always submerged. That's the second issue.
An oil feed line would have to be plumbed into the oil pump pickup orifice and well-sealed. That extended pickup would then be plumbed into the bottom of the external sump. There'd be lag time to get oil flowing and I've not a clue how long that would be. As long as the pickup is always submerged, it's unlikely to be a problem.
That said, the one issue that'll fight you to the death is sheer lack of cubic real estate. The external sump would have to be very deep, and with a small cross-section to maintain oil depth. Can't see where, say, a 6" deep external tank could be safely located, beneath the engine.
Only way to accurately gauge how much oil is needed to keep the stock oil pickup submerged is to pull the top end, set the engine at the same angle created while stunting, then pouring in oil until it reaches the sump area. Best guess, close to 2 quarts. In any event, the oil level would be above cylinder height, while the bike is being ridden normally, i.e. on two wheels. The windage would be insane. There's a good chance of hydrolocking.
As for those who "get away" with extended wheelies, they really don't. It takes a while for the oil film to go away and these engines don't need much oil pressure. The top end (valvetrain) can survive for a surprisingly long time on residual oil. The lower end might get by on splash oiling...for a time. However, the cylinder probably won't get uniform splash oiling, the crank assembly will starve and everything from the crank-up will lose cooling. Oil also acts as liquid coolant in these motors. What's actually going on is that they don't run long enough to reach the point of rapid, catastrophic, failure. I'll guarandamntee that wear is accelerated on some parts, shortening service life of the motor. Given enough mileage, they'll be rebuilding sooner than would otherwise be the case. And that is, perhaps, another key piece of this puzzle. Stunt riding doesn't put much mileage on the odometer, so maybe burning-up thousands of miles between rebuilds doesn't matter...to them. Maybe it won't to you, either. You're the only one who can answer that one.