You can use BuySellAds units with Google Adsense as backfill.

For an example of this you can check out our Discourse instance: http://forums.hummingbird.me/ (ads may not load on the initial pageview because of a timing bug I have not fixed yet, but try clicking somewhere and back to the homepage).

This is the source code: https://github.com/vikhyat/discourse-bsa/blob/master/assets/... (Ember.js)