Changing rest pose from T pose to hands by sides

Hi All, I've got a bit of a conundrum at the moment. I've got a character in a T pose for the bones setup but now I want the main pose to be with hand by his sides. However 'return to pose' obv seems to flip the hands back to the T pose. Is there anyway to change the 'base' pose so CA recognises the pose I designate as the default 'rest pose'? Apologies if thas has been answered before or obvious but Im struggling to get this to work. 

Currently Im using 'hold in place' and manually returning the hands to to their 'rest pose' and makes the wrists super floaty. 

Many thanks in advance. Bone setup attached.