Dan - I would take the ground from the link boards back to the Star Ground. I also think you should wire both boards to a DPDT switch.
I've left the 'output' sections on the ratio boards empty.
I've finished mine and it seems to be working fairly nicely on dual mono mode. I have one issue when I use it in Slam mode. In this mode the meter works fine on the +4 setting, but when I move it to show GR the meter goes hard right. It responds to the input - so it shows gain reduction, it just seems to go hard right whenever it returns to zero. The meter shows GR correctly in all other ratio settings. Any ideas? Is this expected behavior.....? I've got a feeling it might be....
Also - not quite sure how to verify that link mode is working correctly. When I engage it only one channel seems to be working and it the amount it is compressing reduces significantly. The GR meter on the other channel is unresponsive.
Also, I find moving the release affects the meter GR display by 3-4db. I've read somewhere this might be due to the diodes used in the circuit. Does anyone have any more info?