Ok .. it looks like this post explains it. To keep the fx loop engaged, it looks like a trs inserted without the ring grounded will do it.
So I should be able to just switch out the TS plug for a TRS plug and still control the gain using the tip and keep the ring open to keep the loop engaged.
I plugged in a TS foot switch I built and it controls the gain boost/channel.
I tried the fx loop with this cable plugged in and it didn't work. I thought the TS plug would short the ring on the jack to ground and turn on the fx loop, but no dice.
Is this resistor needed for it to work...