Inserting row into forum discussion

Hi everyone. We've been using vanilla on www.streettalk.co.nz for a couple of years now and originally we had hacked a source file to insert some adserving code into the forum discussion so that ads were appearing within the content on the page as opposed to in the sidebar or at the top or bottom of the page. Before I get elbows deep in php code again, does anyone know if there is an add-on that will either insert a row with an ad served in it or something that will insert an ad into an actual comment. Would ideally like to be able to specify comment number, or even better, just before the last comment. Thanks