I wonder if the Pig On the Wall name mightn't have come from the bricklayers' expression "pig in the wall"
Check the wall of the pub to see where there is a "pig" in the brickwork From a bricklayer friend of mine .
"The pig in the wall is when two bricklayers build their leads to a certain level but one of them has bedded too high causing the courses to become unequal in number at each end. If it happens it means shame and ridicule for whatever set of Bricklayers it happens to - which will follow them from site to site thereafter."