working on https://code.google.com/archive/p/sibexport/source/default/source to try and understand .sib format
The code for the original plugin was dumped out of the .plg file using strings -e=l since I don't have access to a copy of sibelius.
This code it quite old so presumably won't work well for the newest versions of sibelius. However I hope it will give a basic idea of how the file format (or at least the format when loaded into sibelius) works.