I have seen people using walking devices that also have seats. The most common type are those rollator walkers, but on Monday I was in line for Jingle Cruise and a woman was using a cane that converted into a tripod seat, and sitting down whenever the line paused. Google "cane with seat" and you'll see what I mean. I don't know Disney's official position on that, but since it's designed primarily as a mobility device that allows you to take breaks, it might be exempt from the no-chair rule.