I was very interested to read about the new Roon ARC. I am going on holiday tomorrow and pleased to be able to use Roon for headphone listening. The main advantage over using Qobuz directly is being able to use the headphone DSP settings. As the ARC app has no facility for DSP, can I pre-set it before I go away? If not I can see very little use for ARC

No - DSP can be used on “traditional” Zones, but not yet on Roon ARC.