There actually are companies that do ad targeting into email newsletters (industrybrains comes to mind), but I think you're kind of missing the point by doing it that way.

You've got a list of people who are very interested in a particular topic. Hopefully you know some more about them too (read: you should be asking questions on the signup page that advertisers want to know the answers to). You can probably tell what kind of ads are going to do well with your audience better than a text analysis algorithm. And ideally you want advertisers who understand the value of an email newsletter over other media, not ones who are willing to appear next to certain keywords, wherever they may be.