For information I developed a Saas application working fine with MODX.
Currently the application, named "Itchikai" is not fully localized but it's planned.
Don't hesitate to contact me for further information.
Itchikai / Web accessibility extension
Démo (in French, next in English) :
Additionally, Itchiweb.com is also able to set-up any WCAG 2.0 / Section 508 MODX website