One thing you can do with current WeBWorK installations is to use MathJax in place of jsMath to display mathematics. MathJax has very good accessibility options --- see
The rest of a WeBWorK problem is standard html so I would hope there are good options for that.
To use MathJax in place of jsMath, you need to install MathJax on your server and then edit the global.conf file. Sometime in the near future, MathJax will become one of the standard display options for WeBWorK.