If you want to get your score files out of the Sibelius data jail, and you don't like the MusicXML data model (or want to pay $200 for the Recordare Dolet plug-in) seriously enough to do some programming, you don't have to struggle with programming a Sibelius plug-in in the deplorable ManuScript language. The sibxml package allows you to quickly dump everything from a Sibelius score in XML, and provides a set of easy-to-subclass Python classes to let you do whatever you want with it. Runs in Python 2.x (x >= 5), supports output from Sibelius 4, 5, 6, and 6.1. DTDs are available, W3C Schema in the works. Contact me for the code.
Python, Mac only for exporting, XML output is platform-neutral
(Requires Sibelius for the initial export step)