A great piece of code that is very easy to install is here:
This you can customize to show only to guests (or by removing that line) to everyone.
It only posts after the first post and the last post of a thread, so you won't violate Google's TOD.
Its not a product or a plug-in, just a couple of lines you add to your template.
Perhaps a vB coder can merge the two into a good working product.
https://vborg.vbsupport.ru/showthread.php?t=162517