The footswitch simply activates the send/return jacks, rather than bypassing the whole loop circuit - that's why you need to bridge the loop with a patch cable, on the amps with a selectable loop, if you want to run any of the loop functions (Solo etc) with no effects in the loop.
Or, you could copy the circuit from one of the non-selectable-loop amps (eg Single Rectifier) and use a switched jack in the Send, with the switch terminal connected to the return - that does the same thing as the patch cable, but inside the amp.
It depends on how much soldering you want to do
.