In this modern age of thermoplastics and alloyed metals, it is becoming a regular practice to build into some sheath designs a compression point that either grips the blade hard enough via friction to hold it in place in almost any configuration, or to have a detent of sorts that either a bump or a depression (it can go one way or the other) would engage, kinda like a sear pin, if you can picture that. You could do that with old sheaths by engineering a latch mechanism, usually with a pin transversing the sheath with some spring mechanism behind it.

More appropriately, most standard leather sheaths would have a retaining loop that would snap or toggle or otherwise secure into place once the knife is inserted and designed so it would catch the choil or the guard. This is more my style anyways, and there are pluses and minuses either way. I don't reckon I would care for carrying a big knife inverted at all, seems kinda awkward and inappropriate to me. Even with a retainer of some sort, I would still worry about it falling out under rugged transport, and at most likely the most undesirable moment.