search for: theora_encode_ctl

...Ogg layer, but you'd have to know what you're doing. There also may be some mucking around in the internal theora state that would work, but you're on your own to figure out how to do it (and be prepared for it to break later). As an aside to others on the list, the theora-exp API has theora_encode_ctl(), which could set this on the fly. Not saying that that encoder is ready for public use yet, just the API.