Thinking more...

There are 2 ways to run the script:

1. Run banshee with parameter: --run-scripts your_boo_scripts.boo
2. Place the scripts in ~/.config/banshee-1/boo-scripts/

If you are familar with boo, it is not hard to add more functions for Banshee.
Of course, you have to understand the source code of Banshee. Beside the source code, you can refer to the official extension provided by Banshee.