Sorry, I should have been more clear. I'm looking for a way to do it like an addon because the HTML will change for every member. I have the template for it built, which I have to pass additional parameters to, but haven't been able to figure out how to add it yet. I tried making a event code listener at the 'message_content' hook and changing the value of '$message.signatureHtml', but it doesn't then carry on to the signature part of the 'message' template.
I know it's working, because if I set the script to simply echo the altered '$hookParams['message']['signatureHtml']' to me it looks right. However, what is echoed to me isn't what is showed in the actual signature area of the posts.
I'll give it a look, but I think that addon adds the ad under the 'messageMeta'. Thanks, though. The more I look at this, the more I think I'll probably just have to edit the message template, which I really don't want to have to do.