In order to raise an issue or proposal in regards to the HTML5 specification you do not have to be a member of the W3C HTML Working Group (HTML WG). Anyone can simply enter a bug into the HTML Bugzilla. If the proposal or issue is rejected by those that control the specification and the issue is related to accessibility, you can refer the issue to the W3C Protocols and Formats Working Group (PF WG). Probably the best method is to subscribe to and send an email to the WAI-XTECH mailing list.
I would suggest that you take this course of action if you consider that the issue raised to be substantive and it has not been given appropriate consideration due to lack of accessibility expertise or understanding of the obligations of the HTML WG to ensure deliverables will satisfy accessibility requirements.
The PF WGÂ is responsible for ensuring accessibility considerations are taken into account in all specifications produced within the W3C. Â If the PF WG considers the matter substantive they may formally request that the HTML WG, not just the editor, reconsider the matter. This may lead to the HTML WG as a whole having a vote on the matter, that is up to the HTML WG Chairs to decide.
By following this course of action you at least guaranteed that the issue will be discussed by a group within the W3C that has expertise in relation to accessibility and the web. It will also be considered employing the W3C consensus process, which is not a process currently used in practice within the HTML WG.
W3C Member Organisations
You can also bring matters to the attention of W3C member organisations, so when it comes time to review and vote on the HTML5 specification before publication, those organisations can make an informed decision about whether the specification takes the accessibility requirements of their constituents into account.
W3C members include organisations such as the Royal National Institute for the Blind and vision australia who represent the interest of people with disabilities, so you can voice your concerns with them directly.