I used a good ol' mechanical 4PDT relay to disable the brain's outputs to the dials through the dacs, until the brain evaluates that it's ready to take control. Leaving full direct control from the dials. But that's probably the major difference in our setups, how I split that circuit it two pieces, used the relay, and retained the dash dials for use ( I had to pull 2 resistors in each leg on the control head and put equivalents on the interface to make that work). I know you needed the dash real estate. But we both used the turn on, "I'm ready" output idea to solve that delimma.