My guess (Similar to the first post on all pages mod) is that you add an extra column to the thread table and store who created the thread inside it.
Edit - Silly me...thread starter is stored in the thread table...not sure if it can normally be accessed though, maybe globalize it and then use a conditional?
Access the variable inside the postbit with a conditional.
Edit again.. Solution!
Find in postbit
HTML Code:
<!-- sig --><div>
__________________<br />
$post[signature]
</div><!-- / sig -->
Replace...change code to fit needs
HTML Code:
<if condition="$post['signature'] OR $thread[postuserid] == $post[userid]"><!-- sig --><div>
__________________<br /><if condition="$thread[postuserid] == $post[userid]">
testing..
</if>
$post[signature]
</div><!-- / sig --></if>
The if conditions may seem redundant since there's two...but the catch is if the user doesn't have a signature it won't post him as the thread starter regardless of if he is or not.