I can only really give you a general answer, since every engine is different. Yes, you're correct in thinking that with a wet sump engine, gravity is what returns the oil to the pan in most scenarios. You'd run into problems with oil pooling in certain areas like the head, as well as it running low in the pan. Even with the correct amount of oil in the pan to start with, the oil pickup might not be submerged to begin with depending on the engine and which side it's placed on.
If you haven't selected an engine for your application yet, you might want to look at a dry sump setup, as it's likely a better fit. In a dry sump, the oil is stored in a remote reservoir instead of in a pan connected to the crankcase, and is transported back and forth with pumps. One of it's benefits is, "Prevention of the engine experiencing oil-starvation during high g-loads".