This is a 1936 home in Virginia with BX wiring and metal switch and outlet boxes. We went to replace original three-way light switches located at the top and the bottom of the stairs controlling a hall light and found that the neutral wire from switch at the bottom of the stairs is connected to the metal box at the top of the stairs. There is no other neutral in the box at the top of the stairs to connect it to otherwise. With the new switch, connecting it to the metal box is only way to make it work. Is that o.k.? It seems to have worked for 80 years.
No, you cannot bootleg neutral from the very rickety ground BX provides.
It worked for 80 years because normal switches don't need neutral. You are introducing a new wrinkle: a smart switch that does. Here is how classical 3-way circuits work. Remember current flows in loops, and the route may differ, certainly as will the color codes.
Note that neither 3-way switch uses neutral, and the "remote" 3-way doesn't even have neutral anywhere near. Remember: installers are stuck with the colors that cables are manufactured with, so white wires may not be be neutral. Travelers are the ones that land on brass screws, they can be any color. I prefer to mark wires with colored tape to indicate their actual function, but apparently, I'm weird.
The problem is you want to put the smart switch somewhere it can't go. You need to rethink.
Generally, you handle this by carefully selecting a smart-switch master that goes at the switch location that does have a neutral. Then you either accept a plain switch at the remote location, or use a matching smart switch "remote". In the latter case, you re-task the 3 wires to be whatever you need for the remote to work.