I think it was Nicola that told me how to do this a few years ago. If you aren’t using the IEM monitor strip for anything you can route it to the headphone output and assign everything to IEM, then have a mute button for it on a softkey.
If you search through my profile for threads I’ve created you may be able to find it.
The advice I gave to Chris was to use an IEM strip for the control of headphones. You can assign all channels to the IEM strip (like you would assign channels to a DCA), set the ‘IEM to Headphones’ option in Setup/Audio/PAFL, and use the IEM fader and Mute to control your headphones. However I don’t believe the IEM Mute can be assigned to a SoftKey.