We'd like some sort of <span> tagging to separate the various elements in a machine-readable signature. This is the HTML output of the wikitext parser function of T230653: Use a parser function to encapsulate signatures. (T230653 simplifies parsing signature information from wikitext; this task would simplify parsing signature information from rendered HTML.)
I'd suggest trying to use web standards whenever possible. As a starting point, the [<time> element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/time) around the comment time, with appropriate microdata tagging to indicate that this is a commentTime. There are similar schema.org attributes for other comment-related data --- see for example UserComments and Comment --- although we probably shouldn't get *too* carried away. The semantic anchor for a user can either be their user page URL (ie, https://en.wikipedia.org/wiki/User:cscott) or by their stable user id (ie, https://en.wikipedia.org/wiki/Special:Redirect/user/173490 ).
As a base case, the current HTML emitted by a comment ending in ~~~~ in wikitext (with a sample comment text preceding) is:
<dd> The logo was never really non-free, as far as I can tell... <a href="/wiki/User:Cananian" class="mw-redirect" title="User:Cananian">C. Scott Ananian</a> 18:07, 29 March 2007 (UTC) </dd>
So something like:
<span vocab="http://schema.org" typeof="Comment" class="mw-signature"> <a href="/wiki/User:cscott" property="creator">C. Scott Ananian</a> <time property="dateCreated" datetime="2007-03-29T18:07Z">18:07, 29 March 2007 (UTC)</time> </span>
We should also consider the issues raised in T120409: RESTBase should honor wiki-wide deletion/suppression of users as well. Parsoid's (former) semantic tagging about users is visible in 89f0eedf78173a0dcf343d5ab223c3da223c9201.