how can I set a new rest pose to a character that was rigged with Auto Rig Pro (arp)?
Bring your character into the Pose you want to set as rest pose, press ALT+H to reveal every bone hidden by the Rigger, select all Bones with A, press ctrl+A and then “Apply as new rest pose”
Hope that helps 
Cheers
its don’t work well with the Auto Rig Pro, becuase it has a lot of extra bones and drivers and constraints and if i do it, it brake the rig
hey gilad sorry to bug ya. Did you ever figure this out? I’m having the same issue if I set a new rest pose it breaks my rig and controls. Thanks in advance