I am doing something *similar*, except with video content. I took a different route that I'll share because it might spark another creative offshoot.
I have two versions of the same site (in development as I write this ... so these links point to my current site which I'll soon be replacing). So:
http://www.learnvisualstudio.nethttp://members.learnvisualstudio.netWhy two versions of the same site when s2Member will adequately handle a free and paid version using the same install? Two main reasons: flexibility and simplicity. 'Flexibility' to make one site purely promotional in nature, and 'Simplicity' to avoid the situation you described -- having this logic sprinkled into every new page of content I have to create.
The 'www' side (my pure WordPress install site) will have 1 minute preview videos, the sidebars will promote the paid membership, and will have a popup with an offer for a free trial membership.
The 'members' side (my s2Member-powered site) will be where I take payment, allow login ... all the things that s2Member does so well. Here I have the full versions of the videos and other downloads / resources. I also have the free trial membership here -- a ten day trial that drips about 30 screencasts over 10 days, then creates a sense of urgency by cutting them off at the end of 10 days.
During development I had some Elancers copy and paste content from my current site into new pages in WordPress on 'members.learnvisualstudio.net'. We took a few passes to clean up URLs, links, etc. We have about 1000 content pages we had to convert.
Once the content was massaged, I exported the data and imported into the 'www.learnvisualstudio.net' into XML format, then changed all references of 'members' to 'www' using find-and-replace in a text editor. Then, I imported the modified XML file.
Now, I have duplicate content! Yes, going forward I'll have to manage two versions of the same content, which is fine because WordPress makes it easy to publish.
The benefits:
-- I can highly optimize the 'www' site using caching plugins and such without worrying about affecting membership functionality.
-- I can tailor the 'www' site to SEO, and tailor the 'members' site to upsell and other matters related to members.
-- I can automate the dual publish of a single "article" (video, description, full-text, whatever) using a third-party tool which will allow me to publish to multiple destinations.
I've given this months of thought and while there are downsides, I think the upsides for my situation outweigh them. Again, I offer this as another approach to a similar issue. Hope it helps a little.