I don't see how it's realistically possible to type him, but I'll bite.
I'll go with ISTP.
He won his first duel at thirteen. I can't see any Intuitive type being that good a fighter at such a young age. And not just good, but arguably the all-time world's best.
Some of the stories about him have a definite "P" flavor, too. For example, not worrying about upcoming duels to the extent that he carves an oar into a wooden sword right before hand. Very casual, no preparation.
In another, he didn't trust the person he was scheduled to fight, so he arrived at the dueling site the day before and hid in a tree. Sure enough, his opponent comes with a bunch of armed henchmen for an ambush. Musashi jumps out of the tree right on top of them and cuts them down as they're paralyzed with surprise. Sounds like very SP-ish tactics to me.
He did indeed write and paint. Any type can potentially do that though. His book was on tactics, which is right up the ISTP alley, whether it's military tactics, football tactics, or dueling tactics.