to follow up on what Welch, Stephen R. said:
However, I'm not clear on how to do this -- how do I refer to
the message list order of the author index? What other
variables should I use, define, or create?
You need to know which author page a citation is on, but you
don't need to refer to messages in author-page order ever. The
serial number [as used in message page file name] from the main
chronological order suffices for a unique id for the message and
you can use that in anchor names to get to the right entry once
you are in the right HTML document. Even when the page does not
list them in that order.