I wrote up an article on how to have a tool load custom code.
I had a situation where a song neededdifferentsettings when used ondifferentmachines and I got tired of manually adjusting things, so I wrote a tool for myself that looks for filerelatedto the currently loaded song, and if found it loads it and executes a configuration method.
http://neurogami.com/blog/neurogami-loading-custom-code-in-renoise-tools.html