How many square foot of coverage per bag of mortar when repointing brick?
Had a few minutes... created a model in SketchUp and found that with a 3/8" wide joint the mortar takes up .55 sq ft per square foot of wall. 1" depth after removing part of the old mortar and it comes out to 0.015 cubic feet per square foot of wall. 

About 0.7 cubic feet of wet mortar per 80lb bag and that comes out to close to 46 square feet per 80lb bag for a 3/8" wide mortar joint.

With a 1/2" joint width yield is roughly 36 square feet per bag.