to follow up on what Earl Hood said:
What resources can I use as the equivalents to PREVLINK, NEXTLINK,
IDXFNAME, etc so that each message can be linked to
next/prev by author as well as by date and thread?
You can't. Messages are written/edited before any other index sorting
is done. To support what you want, mhonarc would have to create sort
lists for all possible sort criteria (an expensive operation), and also
provide a bunch of resource variables to access them.
Some optimization could be done by allowing a preset list of sorting
to be done, but the resource variable list would still be quite large.
If the anchor names in the indices were built by rule from the message
ID, you don't have to have sorted the index lists to write the URLs
that index into their HTML representation. You do have to ensure that
every message has an ID, but I think that can be arranged.
-- Al Gilman