Asked the same question myself and the answer is no - you can only select either the switchable or permanent.
It would be nice though - delays, choruses etc through the permanent (switchable on/off via rack floorboard) and a level boost through the switchable to use all 3 channels as rhythm/lead via the loop button.
Well, that idea is out. Just as a side note FYI, I called Mesa yesterday to find out if there was stereo loop mod to the (A) models, similar to the (B) model loop setup. No such luck. I still love my (A), though.