Where this matters is if the NEMA 10-30R were served off a subpanel with isolated neutral and ground. For an appliance -- especially one that makes contact with earth ground, like a range sitting on concrete, for example -- this could allow stray currents from the neutral to return to ground through the chassis. It really requires another failure somewhere in the system, but I've seen enough failures to create that condition.
Just like the NEMA 6-20 thread, do NOT do this in any permanent installation; it's a very bad idea and won't pass code. A "cheater cord" as described will work, but may have some unintended side-effects (like minor shocks in certain cases).
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We're getting mixed up on terminology. I'm using specific terminology for a reason -- safety... stick with me here as the "neutral" and "ground" are NOT interchangeable, which is why I'm making sure the caveats of this whole exercise are understood.
The "neutral" conductor is the current return conductor for 120V needs on a center-tapped 120V split-phase system and is referenced to earth ground. The "ground" conductor is the equipment safety grounding conductor and is used to ensure there is a very low-resistance path for equipment chassis to safely return current to ground in case of a system failure.
The "ground" conductor is for safety only and is only intended to carry current in case of a failure so that 1) humans do not become a return path to earth ground for currents, and 2) overcurrent protection devices can open in case of a short. It is absolutely required in today's world for safety reasons.
You're correct in stating that the NEMA 14-50R will still work if the neutral pin in the receptacle is unconnected, *for charging the Tesla*. But a 14-50R is not unique to Tesla, and you must not assume others will have the knowledge that you have about that pin being unconnected. And finally, the NEMA 10-30R has no ground pin in it -- only neutral -- so tying the "ground" on one end to the "neutral" on the other is something that has to be labeled. That's why I say if you do it, 1) do it only in a cord; 2) mark that cord from here to heaven and back as I state in the preference order above. It is expected for a NEMA 14-50R to have a ground and a neutral connection. If you sell your home, or are incapacitated or die (perhaps at the hand of your own electrical invention), others must know what is present so it doesn't create a hazard.
Electricity kills and just because it works, doesn't mean it's safe or won't burn down a building. It may seem like I'm arguing about technical details, but unfortunately I've attended the funerals of people who die from these types of mistakes.