I'm very late in responding here, but maybe someone will still see this... Instead of a branded subreddit, do you know what I (as an advertiser) really, really want? A branded novelty account. I want to have a novelty account where I can post comments that are relevant and engaging with an occasional link back to my site, and I'd like for the users of reddit to somehow know that I am a sponsored account (perhaps through a "sponsored account icon" next to my username). My comments would be subject to downvotes and upvotes just like any other comment.
If you, or someone else at Reddit, would like to discuss more, my contact info is in my profile. Cheers.