Note that if you expect the PM to be effective, you need to default pm message box to on (Edit: this is a template mod), especially if the people only registered to participate in a single thread. Also, you should just alter the postbit_signature template to restrict the display size of sigs, which in most cases will encourage people to make their sig fit in the box. That is what we have started to do at Zelaron. The code is pretty simple. The template should look something like this:
<p>_________________</p>
<div style="width:300;height:125;overflow:hidden;">$post[signature]</div>
Since this is a template mod, there should be no issue with using this.
--WetWired
P.S. So, what are your impressions after seing the postbit "live" ?