@dcrainmaker, I have a possible workaround for doing different and custom workouts with the Group Workout as a portal
In your case, run the custom on mobile or another desktop, while running a regular workout on the GW TR session. Unpaired, you cans still hit play and let people follow that workout or they can do the hack.
You could also use their team function, to share your custom workout, so people could access there, and run with the hack on their side.
All a fair bit of work to setup, but it should work until TR unlocks custom ones natively.