Would you consider to increase the length of the data in the RSS feed, maybe to the maximum possible value so that all posts are fully available? (Maybe for subscribers only; this would probably require some tweaking of the SMF code though, and I'd have to teach rss2email to send a session cookie...)
I fully understand that you may not like the idea because the more people read the forums by RSS, the less ad revenue you'll generate. But from my own experience though (running a forum which offers an Atom feed), relatively few people use these options at all.
I will look into this - we are going to update the forum software soon - so it is likely any changes will be after that update.