
What Would Work
What would work would be to create a handle that attaches to the lens mount so that what you hold is actually not the lens or the phone, but rather the mount that holds them together, preferably made of something like nickel plated tool steel. Sure that is expensive, but anyone buying a phone with the intent to use it with lenses that cost several thousand dollars would think it was worth every penny.
So you make a plate with something that looks like a larger lens mount. The adapter attaches to this, and the adapter has the actual lens mount on it, with all the electronic pass-through's that are needed. The handle can also have things like a physical shutter button! An additional advantage of doing it this way is that you can reuse most of the design to make another device that uses some other lens mount. So you can make a Leica phone, a Nikon phone, a Cannon phone, and maybe even a phone that is designed to use cinema lenses. Sure they would be expensive, but the market is not the phone buyer, but the person that already has the lenses and wants a compact, quick and convenient way of getting professional grade content online.
Honestly I am somewhat surprised that this does not already exist.