you can do this by using a conditional in your postbit (or postbit_legacy template) you need to do:
Code:
<if condition="THIS_SCRIPT == 'private'">
<!-- private message layout goes here -->
<else />
<!--normal postbit code goes here -->
</if>
on my board I copied the postbit template into the first section and the postbit-legacy in the second so there`s a nice change between the two..