the molasses in a stout is not a bad idea at all I would shy away from using it to prime with,
a. the bottle conditioning will take a very long time.
b. with the introduction of large ammounts of sugar in the bottle there are risks of bottle bombs, gushers or leakers
Molasses is still used today in commercial brews
Theakston Old Peculier Old Ale
I would also search google, using beer and molasses, I found several ideas this way !