Hi @charleswalter, apologies for the radio silence - Rails Girls finished end of last month and we got back to our 'day jobs'.
Just so that I'm understanding correctly, what you'd like is that custom targeting in DFP will automatically target, as a key: category and value: whatever category it is. For example, if a person goes to a page under the Jennifer Lopez category on your site, then all ads with the targeting key of "category" and value "jennifer-lopez" will turn up. Likewise, if someone goes to a Postres y Delicias page then the ads that target delicious desserts will then pop up. Is that correct?
If that's what you're after, sounds like a good feature. We actually thought about putting it in during Rails Girls Summer of Code but time ran out before we could get on to it and there are a few things we'd need to do to get this to work. Feel free to create an issue here https://github.com/team-melbourne-rgsoc2015/discourse-adplugin and if enough people comment on it, I'll look at allocating some time to doing this.