There are big privacy concerns with user tracking. Most people with disabilities do not want to be tracked, as it means they have to disclose they have a disability in a space where otherwise they can go without waving a big white flag (or cane) that says "HI, I'M DISABLED!" The best thing we have is the WebAIM Screen Reader survey, in which users self-report what browsers and assistive technologies they use:
http://webaim.org/projects/screenreadersurvey6/
One thing you could do is add event-level tracking in your own analytics to measure accessibility features such as skip links and keyboard-interactive controls. But that would be specific to your own application.