If I had no problem with devoting the time and money, contributing to the kernel (especially in a topic as obscure as making the extra buttons work on a 20-year-old laptop) is at the top of my bucket list, and I am definitely going to be doing it in the near future when my calendar clears up a bit. Exquisite write-up and OP's simple writing has a motivating ring to it, and I'm now on the local used marketplace lookin…
I feel most laptops still don't work completely out of the box with Linux, so you don't have to hunt for old hardware. Maybe you won't find an issue as simple as fixing a button, though.
Every laptop I've used with linux has had a few non-functioning buttons and keys. I think you underestimate the widespread issue.