For the most part, it can be done with just a simple template mod. Surround the whole postbit in a conditional:
Code:
<if condition="$post[postcount]==1"><else /></if>
Put the old postbit between the else and the /if, put your modified one between the if condition and the else. AFAIK, they only need to use a plugin in order to make the first post appear on each page, and I know I've seen that here at some point. Also, you'll probably want to include a forumid in that condition.