If you're using external amplifiers for Hauptwerk, not sending it to aux input on the organ, plugging headphones into the console might turn off the internal speakers. Or disconnect the internal amp.
Otherwise, you could just use the organ's pistons with empty registrations and not use the stop tabs or drawknobs. It's not recommended to use both on a digital organ anyway because the organ's stop states end up differing from Hauptwerk's when the pistons change them in both places.
For example, a piston may turn on some of the Hauptwerk virtual organ stops while turning the same stops off on the console because of the piston's local registration setting, leaving the stop tabs unlighted when the VPO has them on. Then pressing a stop tab tries to turn on a stop that's already on, and the result depends on whether it's a toggle or stateful.