Yes. I started with that some time ago, when I wanted to filter some MIDI messages in the sequences I loaded from SMFs. I have not made out yet all bits from it, but I was able to write a dirty program in C with which I would filter those messages. I can provide you with the main blocks of the file as soon as I write them in English. I have that information, but in a very disordered way.

At Lord Jonin's Homepage there were some links to information about the PCG format. At the time I last visited it there was nothing about SNG, but it might have changed. You can check it at:

http://www.best.com/~jonin/x3-web.html

Anyway, I'll post you a draft of the format as soon as I have something readable.