It's a little difficult to discern here, but there's an uninsulated wire in between the black and white twisted pair along the backplane buss. According to my scope, the bare wire is the audio feed line from the independent cards to the master summing section. Visible are my sloppily hand aligned holes.
The idea here is that since all the cards output direct to this buss wire, I can get away with installing interrupting jacks (Switchcraft 12A) so that the card feeds the tip directly, which is shunted to the combined buss when nothing is plugged into the jack. I've opted to only go so far as metal work on the 11/12 hole, as I don't have a card for that pair. So yeah, I'm leaving a HOLE in the case. Never know when it might come in handy.
Functional, though the recovery amplifier provides for considerable gain, so direct patching the two input card has a much lower level than what is available at transformer output. I guess that's what booster pedals are for.. HAHA.