Hi Steve
As you already know, if a page is not crawlable it's not indexable. I don't think there is any way around this without changing the strategy of the site. You said, _"We have a number of open landing pages but we're limited to what indexable content we can have on these pages". _Is that limitation imposed by a legal requirement or something like that, or by the site owners because they don't want to give free access?
If the marketing strategy for the site is to grow the membership, then as it's providing a content service to its members then it has to give potential customers a sample of its wares.
I think there are two possible solutions.
(1) increase the amount of free content available on the site to give the search engines more content to crawl and make available to people searching or
(2) Provide a decent size excerpt, say the first 250 words of each article as a taster for potential customers and put the site login at the point of the "read more". That way you give the search engines something to get their teeth into which is of a decent length but it's also a decent size teaser to give potential customers an appetite to subscribe.
I hope that helps,
Peter