This really should occur in the buffer of the client. Assuming that the soundbridge is capable of maintaining three buffers. One buffer for each stream of the eventual mix and the output buffer containing the blend of the two.
Post the request to the Roku guys. I’d like to see it too but it is probably too far off from what they’ve already built for it to be on their list any time soon considering how many firmware problems they’re having with people simply trying to operate the device normally.