short answer no
the only type of "free" advertising you could do is a flier campaign or something of that nature... Or in the banner like you have, but afaik there is a strict "no advertising other forums" rule on this board (I don't know and personally don't care, so no worries here).
The only other way would be to create some sort of buzz about the site that would carry it into the media somehow...
However, the best advertising you are likely to get comes by method of word of mouth. For some reason, your forum has to offer something people don't get here or anywhere else.
for instance, I am likely in your target audience, as I am an active board member of just one board. With limited time to allocate to this type of thing, why should I go to your site as opposed to this one? And honestly, you are unlikely to convince me of why I should, but if my friend user X happened to say "Hey timbo, check this out" I probably would...