With the engine upside down, it is harder to get fuel into the engine than to keep it out. Fuel will flow downstream and drip out of the carb like Kirk already pointed out. Syphoning will flood the upright engine, not the inverted one. To avoid that, the plane in readiness should be kept slightly nose-up, so the carb is higher than the tank top.
The real problem is puddling of the oil that has collected inside the engine. With the engine at rest, the oil will accumulate in the lowest point, which happens to be the plug. Even if you can get the plug wire to glow, there will be a slug of oil inside the plug body that prevents the plug element from lighting the fresh charge. Once the engine is running, the oil is expelled better, and plug blocking will no longer occur.