You may get something better, but let me offer the following two
hacks that I would find acceptable:
1. I can't believe that you are going to start a new list
without sending a test message over the list server. Let
procmail put this message in a mail folder and start the archive
with that message (invoke MHonArc manually).
2. I don't see why you can't leave this seed message in the
archive, but if you want it gone you can use -rmm later after the
list has added other messages to the archive.