My grandmother pioneered for over 40 years until her late 80s. After an illness, she went on the disabled list, but still got high hours until around 91. She walked from her home to the Hall - and this is in a major urban area where drug dealers hang-out on the corners only a block from her home. While she got rides on meeting nights, the congregation let her walk to the Hall - something my family didn't know she was still doing after we left that congregation. The only reason she isn't out now is because of another major illness and failing memory. But my folks still had to go over for a while and stop her from going out after she got sick this last time.
Letting the elders know of you concerns sounds like a good idea, especially if you ask them to encourage her in alternate forms of service. I like the disappearing cane trick even better - though it could possibly hamper her getting around at home. And if she's that determined to go out, she'll likely find a way around that. You may be able to only slightly diminish the risks to her if she insists on going and she enjoys doing it; hopefully she's not being pressured, though she's likely being "encouraged" by commendation from other publishers.