An issue came up in our game which I hadn’t thought about before…
If a character is restricted to leather armor or lighter, does this mean armor with encumbrance 2 stone? Other armors can be constructed “light” and magic decreases the encumbrance of armor, so it might be possible… Elven chain, for example?
For game balance, the restriction is determined by the non-magical AC bonus rather than the encumbrance penalty.
That said, I offered rules for high-quality armor elsewhere. Ther'es no reason the "quality" couldn't refer to a change in material. You could define "elven chain" as the equivalent of non-magical +1 leather (encumbrance 2, AC 3, cost 200gp).
"For armor, the magic increases AC and decreases encumbrance. Some quick math similar to the above suggests that armor with a non-magical +1 to AC or -1 to encumbrance should cost 10x as much, and with both 100x as much. So non-magical plate mail with +1 AC (AC 7) but 6 encumbrance would cost (60x10) 600gp. Non-magical platemail with AC7 and 5 encumbrance would cost 6,000gp."