You would basically have a chess computer that you communicated with entirely through your butt.
This would work, even if you played in a Faraday cage since the communication and computation is all internal.
The Nogasm is a pleasure denial project that uses pelvic floor contraction frequency, as sensed by pressure output in an air filled cavity, to prolong play.
https://github.com/nogasm/nogasm
There's even a upscale ESP32 based version of it called the Edge-O-Matic, complete with its own Flappy Bird implementation.
https://maustec.io/pages/edge-o-matic
Output from either of these devices could be used for that, though discerning between voluntary versus involuntary muscle contraction would be challenging.
Your solution would probably work, I guess. I sadly don't know anything about measuring sphincter pressure.
You could also have a pressure sensor in a shoe or something that interfaces with the buttplug via bluetooth, or even a small wire. Though, I could imagine competition security looking at feet but drawing the line before checking butts. So, the chances of getting caught could be higher with my approach.