A week or so ago I posted a picture of a patent for a device used to track the state of a baseball game. From the same source here's a patent for a bat from 1894 by one Charles Jacobus that allows the hitter to insert weights that move.
"The disposition of the sliding b and c within the bat as just explained, will permit the batsman to quickly and forcibly swing the bat for a stroke, and simultaneously with such movement the gravity of the balls b or weight c, will cause either to slide outwardly until stopped by the plug d, thus disposing said weights at the outer end of the bat,increasing the momentum of the moving billet, and enabling the batsman to strike a forcible blow."
Of course Graig Nettles was famous for using a bat that had been hollowed out and filled with six superballs that bounced in the batters box after the bat cracked in a game on September 7, 1974. Nettles pleaded innocent saying he had received the bat from a fan and didn't know the balls were in there. Here's a nice list of some of the doctored bat infractions collected in the wake of the Sammy Sosa incident in 2003. I've not seen this idea described from a physics standpoint so I can't say whether it would really provide more greater bat head speed but on the face of it the idea seems reasonable although the increased velocity would have to compensate for the decreased mass.
It turns out that other have used this basic design. There is apparently a story of a minor leaguer who managed to place a tube of mercury inside his bat, relying on its shifting weight to provide more power. I'd like to know more details if anyone has them.