If I did remember the desired scenario correctly, a solution would be something like this(I have a setup like the following for my bass/guitar fuzz-board…for when I’m using an amp with an fx loop.)
A simple, true bypass, switchable, single loop pedal. Desired fx are placed in the pedals loop, while the pedal is placed in the amp loop. When engaged the amp loop goes through the fx pedals. When the loop pedal is disengaged, the amp loop sees nothing. If this is set up with a Dual/Triple Rec(of the era in question), and the loop select feature s dialed to channel three, then the loop will be only active in channel three…and fully bypassable(via the little loop switch). This is especially useful if it is desired to suddenly “shunt” a chain of fx pedals with a single stomp.
This is similar to my switch, same builder, but different orientation of jacks(on mine).
I know it sounds silly, but if I am right about what he wants, that is part of the solution. The “solo/boost” function will be disengaged though.