In his message of Aug 11 2000, Sean Palmer suggests a new "comment" attribute. There were a few supportive "why not?" responses.
This example shows that it isn't (or: shouldn't be) necessary to pray to the centralized HTML Gods to get your new tags and attributes any more. Just do it: put a schema for your tags in the web, and use it.
This document isn't a strictly conforming XHTML 1.0 document, but it's well-formed and it schema-validates, per a schema I cooked up for XHTML 1.0, plus a schema for the suggested comment attribute.
See the results for yourself (using XSV.
cf greymarket HTML in my XML '98 talk.