Any word on how much (in terms of weight) a mule can pack?
Depends on the mule.
General Crook could get his to carry 400 lbs.
During the Civil War, Confederate General John Hunt Morgan traded my great-great-grandfather the mule for his horse (a cross between a Thoroughbred and a draft horse). Neither side would let him join up due to his huge size (he would make an awfully big target), so he spent most of the war riding back and forth between the two sides as a neutral courier and a mailman.