How does someone like me, a PhD candidate who has a background in Human-Computer Interaction, contribute? Or the accessibility researchers I collaborate with who sit one office down and use and love Linux and make their own hacks for Linux to improve its accessibility?

I think part of the problem is a matchmaking (as as you said a PR) problem. I'm sure a lot of the experts you describe would love to contribute some cycles (including myself) but your post is the first time I'm hearing that people like me are needed in my HCI capacity. Otherwise I would have assumed there are plenty of programmers better than me available so why bother?