According to the info from the scene release of MLB 11 the workaround method is the same as Dragon Age with .pkg files included. On a side note there is a working copy of Bulletstorm from Europe finally.
Tested on 3.55 kmeaw CFW, since game requires 3.56 you will need to do the following:
1. Install pkg file included in pkg folder
2. Open up Backup Manager (tested on Multiman " --- " 1.15.03).
3. Change Param Sfo file from 3.56 to 3.55
4. Load it up with a game disc in drive
5. Play the game