Is there enough clearance between the seat rails and the AXS battery on the Reverb AXS seatpost to use a saddle roll or bag?

Yes, a saddle bag can be used as long as it attaches ONLY to the saddle rails and not the upper post of the Reverb AXS seatpost. The bag must not rub on the seatpost or contact the seatpost where it can become pinched during compression and cause damage to the seatpost.