Im with Tim on this one
use if statement in postbit / postbit_legacy
use following code in showthread.php
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in the templates you got a bit more work
You would need to edit all instances of $post[username] and use the <if condition="$show['asanon']"><class="smallfont">Anonymous<else />$post[username]</if>
im guessing youll also need to use the if statement for userid also.
Test it then test it again because your going to have to use the if statement to replace replace info that would relate to the user
This is untested since there are way too many postbit changes for me to bother sorry.
If it doesnt work check your <if condition statements have been closed at teh end </if> regardless of the </if> already existing before.
Good luck hope it helps.