I just took a look at this and I think I see how it can be done. I would hook the input Jack to the informant board as normal, wires for ground, sw and out hook to the relay board as normal but BP hooks to the input Jack connection on the relay board, leaving the in pad you would normally use unused. Out goes from the normal spot on the relay board to your output Jack.
Basically you’re just running your buffer signal as the input Jack on the relay board but running your actual input Jack to the informant. I haven’t had sleep so I might be dead wrong so someone else should weigh in but I think this is the way to do it.