Were to put reply links

First, if you only want the replies to go to one email address, you will
not use the &Cc=$FROM:U$ part.  Again, if I take what you said strictly
you don't want to bother with an In-Reply-To: header and you can leave
that paramter off, too.

So I interpret your plan to involve two different kinds of URLs:

  mailto:specific(_at_)email(_dot_)address?Subject=defined%20subject

  mailto:specific(_at_)email(_dot_)address?Subject=Re:%20$SUBJECTNA:U$

You need to make each of these URLs the HREF attribute of an anchor in
you message-page resources, such as BOTLINKS.

Look at

  http://www.access.digex.net/%7Easgilman/dev-access/2/headless.rc

for how I did it.

Wrap the anchor in text that tells the user what they do if they 
exercise this link.

--
Al Gilman