I have a similar “problem” with my campus: most content is behind membership wall, so if someone is searching for something that I HAVE, they wont find it. I found a way around it and am getting someone to help me set it up. This might or might not be suitable for you.
– i am creating various pages (in my case, it will be one page per tutorial) that are totally hidden from navigation
– those pages are keyword rich, for SEO purpose but also a “selling pitch” for my product
– those pages will be linked together and with related tutorials “hidden pages”, but all will direct viewers to the real sale pitch page
This setup allows me to take advantage of SEO, and search engine ranking without giving the actual content away (i am just trying to convice you that i HAVE the content and that you WANT it!)
Here is one example of such pages: Postage Stamp
Like i said, it might not be the answer for your particular site though, but maybe something you can tweak.