I think that'd be tough. 2ch DACs don't have to  licensing fees to decode Dolby and DTS multi-channel formats. I think you would end up with a very expensive multi-channel DAC that most people wouldn't need. For HT purposes, I don't really think an amazing DAC does much better than one that is just very good. I find that reproducing music can make differences in DACs stand out though.
I tend to listen to most music in surround mode these days.  Yes, I know, OMG the horrors (I don't need any scolding on that... 

!  I listen to a wide variety of music.  Techno/trance sounds phenomenal in surround (especially with the speed of the Maggies)!  Sometimes, for certain styles or older recordings, I listen in 2ch.
I had a BDA-1 for a couple of weeks - it's analog out to my SP.  Sounded great, of course.  That approach completely ignores the DAC part of the SP.  That's the duplication I'm referring to.
So I'm not interested in a 2ch DAC.  I just want the latest/greatest full DAC from Bryston, and a surround preamp to pair with it...  i.e. BDA-2 + multichannel BP26.  Then I can upgrade each as available and not worry about "which is better" and partial duplications of purchase.
Perhaps I'm alone in this product interest - I love movies and music, and really enjoy HT.  I was hoping to learn the Bryston strategy for their surround product plans with BDA.