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@Shanghaied
Right, that looks like the problem - when you tighten the cable in the clamp you do it with the mech at the inner position, and the bend around the spring takes up an extra amount of cable, longer than it would be if the bend isn't there (think pinching up an exposed length of cable with your fingers). As you shift to the big ring, the spring gets out of the way of the cable, and all of a sudden the bend disappears, the cable tension drops again (think releasing the cable you have between your fingers) and you end up having to shift that extra bit of cable too, and you run out of a clicks on the shifter - you reach the end of the ratchet and can shift no further.
I have both Campag band-on mechs and braze on mechs on adapters (Campag original), and the positions of the cages and linkages are basically exactly the same, so I don't think a band-on mech is the solution. The problem seems to be that the place where the cable goes around the BB shell is too far to the right, hence the cable coming into contact with the spring. Now normally if you have a little plastic cable guide on the BB shell, you can try and move the entire guide to the left a bit - Campag BB-mounted cable guides have a little slot for the fixing screw instead of a hole to allow for this, Shimano have two holes. That ought to get the cable out of the spring's way. Though I'm not sure if you have a cable guide down there at all, since you said you have internally routed cables. But if you do have a removable cable guide on the BB shell(e.g. the cables exit the down tube before the BB shell), move it to the left or buy a new one that would allow more movement to the left. Hope that helps