[Lynne]

If you want to add it into the signature, this is what you want to look for in the postbit (or postbit_legacy, whichever you use):

$template_hook[postbit_signature_start]
        
        <if condition="$post['signature']">
        <!-- sig -->
            <div>
                __________________<br />
                $post[signature]
            </div>
        <!-- / sig -->
        </if>
        
        $template_hook[postbit_signature_end]

So, it looks like you could even use a Template Hook for it.