SOLUTION!
So I worked with Adam Tanner and we resolved the issue.
Turns out that you *have* to use the left-most HDMI output from the perspective of looking at the front of the SP3 - that's the one most outboard, furthest from the HDMI inputs.
Obviously it assumes you are using fast HDMI cables - DV needs a lot of bandwidth and *normal* cables won't do the job.
You also *can't* connect a second HDMI cable to the other out port as that will disable DV on the primary.
So, HUGE kudos to Adam for finding the solution. Great job Bryston. We now have a solution and know what prevents it from working.