Although Pan won't send mail directly, it must provide the ability
to farm the work out to another application.  I imagine specifying
which MUA to use will be a preferences option.

Necessary steps:

 (1) add a "Reply" menu option in the post menu

 (2) add a "To:" header line in the compose window

 (3) when the user hits "Send" and the "To:" line is
     nonempty, invoke the MUA.

 (4) when invoking the MUA, if the "Newsgroups:" header
     is nonempty, add something to the email message only
     stating "this is a copy of an article posted to $group".

 (5) if no MUA is configured when we try to invoke the MUA,
     pop up an error dialog and ask the user to configure
     one via Edit|Preferences|Applications.

 (6) Add Edit|Preferences|Applications.
     The Browser line can be moved to here.

     Browser
       [browser-mode-combo                  ]

     Mail User-Agent
        pan-file-entry
        arguments: [mua-args-combo          ]

     Where mua-args-combo has:
        KMail
        Evolution
        Thunderbird
        Mutt
        Sylpheed
        Custom:

     and custom manages an entry field
     just as browser-mode-combo does,
     plus a help button.

 (7) The MUA arguments:
     (mutt) mailto:%A?subject=%S&cc=%C&body=%B
     (evolution) mailto:%A?Subject=%S&Cc=%C&body=%B
     (kmail) -s %s -c %c --body %b %a
     (balsa) -m mailto:%A?subject=%S&cc=%C&body=%B
     (sylpheed) --compose mailto:%a?subject=%S&cc=%c&body=%B