The main problem with that is I've been working in 5.1 from the beginning, and doing a mix in 6.1 would require redoing all the audio from scratch, not to mention upconverting all other 5.1 audio to match. Unless someone can convert the BR audio to 5.1, it just doesn't make sense to me to redo everything for a marginal difference in the sound.

I could, however, export what I'm working with in a higher bitrate for the MKV version, though it'd still be mostly sourced from DVD audio (save for Episode I).