[Harvey]

cause I don't think that's a request of common interest, here a special untested solution for you.
I Assume guests may not post!

get the forumid of the forum Introductions with phpmyadmin:
select forumid from forum where title='Introductions'

Assume this is 4711.

Edit newthread.php
search

PHP Code:

if (!$permissions[canview] or !$permissions[canpostnew]) {
  show_nopermission();
} 


below both occurences add

PHP Code:

if ($bbuserinfo['posts']==0 && $forumid!=4711) {
  show_nopermission();
} 


edit newreply.php
search

PHP Code:

if (!$permissions['canview'] or (!$permissions['canreplyown'] and $bbuserinfo['userid']==$threadinfo['postuserid'])) {
  show_nopermission();
} 


below add

PHP Code:

if ($bbuserinfo['posts']==0 && $threadinfo[forumid]!=4711) {
  show_nopermission();
} 


perhaps you want replace the new show_nopermission(); by

PHP Code:

    eval("standardredirect(\"".gettemplate("redirect_firstintroduceyou")."\",\"showthread.php?s=$session[sessionhash]&threadid=$threadid\");");
    exit; 


then copy for example the template redirect_threadclosed to a new one calledredirect_firstintroduceyou and redesign it.