the Word Detective says this:

Personally, I have doubts about this story. It's a bit too perfect, and the Oxford English Dictionary lists a non-baseball citation for "south paw," meaning a punch with the left hand, from 1848 [which predates the Seymour usage]. That's a bit early for the phrase to have come from baseball, although it's not absolutely impossible.

http://www.word-detective.com/093098.html

1848 Democratic B-hoy, ‘I say, Lewy, give him a sockdologer!’ ‘Curse the Old Hoss, what a south-paw he has given me!’ 1885 Sporting Life 14 Jan. 4/3 They had always been accustomed to having their opponents hug their bases pretty close, out of respect for Morris' quick throw over to first with that south-paw of his. [OED]