That is actually what I thought you meant, I just wanted to make sure before I told you to do something that was not what you were wanting.
This is really easy to do and only requires template modifications (ie: it can all be done through the Admin CP). What you basically need to do is make a duplicate of your current style that you want to do this to and upload it (make sure if uses different replacements and a different template set).
Once you have uploaded your duplicate style and know it is working, all you need to do is edit the forumdisplay_threadlist template (on the duplicate style) and add the text to it that you are wanting to display right below:
<form action="forumdisplay.php" method="get">
Once you have added what you want shown, you need to goto Forums & Moderators and click on modify, then click on edit on the forum you want this to show on. Once you have clicked on edit, scroll down until you see "Style Options", set the "Custom style set for this forum" to the duplicated style and make sure you have it to where it
does override the users custom styles.
You should now be able to see this only on the forum in which you forced the style on. To have different text for each forum, you will need to repeat the above.
While this may be a lot of work, this is the easiest way to do it without hacking your board, there may be an even easier way to do it with hacking, although that it not my area

.
I hope this helps, if you have any questions, post them here at I will try to further help you out.
Regards,
- SaintDog