How to fix sync issues in multi-output device?
The only solution I've found is to use the third party app AirFoil Speakers and use my iPad as the receiver for my "rear channel" audio (the BT Speaker).  I found an article documenting some debug settings that include per-device sync adjustments:  


  To enable the sync adjustment, follow these steps:
  
  1) Quit and re-launch Airfoil while holding down the option key. This
  will bring up the debugging window.
  
  2) Turn the Sync Adjustment debugger to On, and click Done.
  
  Now each remote speaker will have an additional slider below the
  volume slider - that's the sync adjustment. The center is 0 offset,
  and the ends are +/-1 second. Adjustments only take effect when
  connecting, so if you're experimenting with it, you'll need to
  disconnect, adjust, and then reconnect to see how well it worked.


Very handy and works well once dialed in!  It's unfortunate that in addition to the BT speaker already purchased, this solution also needs both a commercial 3rd party app as well as an expensive iOS device, but unless there is an option somewhere within the Mac OS to adjust sync in a similar fashion but for hardware audio devices, this is going to have to do (and I'm fortunate to already have the iOS device).