Well, it would take a few minutes to write such a script. Doing it in a
useful way would be _much_ more complicated.
But most importantly: Most accessibility experts these days think that
accesskeys are usually _hostile_ to accessibility, hostile to people who
need to use keyboard controls when browsing Web pages. Instead of helping
anyone, accesskey attributes interfere with the key assignments that
people are used to. Moreover, even in situations where this can be
avoided, the assignments are basically hidden from the user, and attempts
to show or speak them will generally result in a confusion.
For more info see
http://www.cs.tut.fi/~jkorpela/forms/accesskey.html
(which I wrote when I was more optimistic, but see the notes near the
end, describing the problems).