Okay, i have been following this thread the last couple of days. I'd like to add my 50cts, even though it might not be what you have in mind.
I have built a couple of (passive) SSL summingboxes for other people. The idea is very simple, and it's actually the same as what people do with the Neumann V475 2B summing amps, with the exception that I used 10k resistors instead of 5k1. The summingamps that I have used are SSL SL552 Indepent Main Output modules. They are from the 5000 series, but they are very decent built and excellent sounding summingamps, based on MAT02 opamps and 5534 opamps. They have balanced inserts pre-fader AND postfader. The fader is just a 10kLin pot on the front that controls 2x2 DBX 2151 VCA's that are on-board.
I've sold one of these thru ebay, about 2 years ago, the auction delivered me a very nice stash of cash, and the new owner is still very, very pleased with the summingbox.
And I must say, I think it sounds better than the Neumann. More modern, a bit fatter.
(oh, on a sidenote, the circuitry is somewhat identical to the 9000 pre amp, but it has fixed gain and it uses bipolar caps, and a bit shorter signal path).
So, if the console has got balanced direct outputs, postfader, you can route these into the summingbox.
In case you can't find a 552 module, SL553, SL543, SL557, and SL594 share the same summingcircuit. The options for inserts, fader etc are different on each module, and some use LM394's instead of MAT02's.
Or you can build a couple of 9k pre's. As said, the circuitry is about the same.