Sometime ago I ran into ChucK, which is a programming-language to generate music/sound/audio:
ChucK presents a new time-based, concurrent programming model that's highly precise and expressive (we call this strongly-timed), as well as dynamic control rates, and the ability to add and modify code on-the-fly. In addition, ChucK supports MIDI, OSC, HID device, and multi-channel audio. It's fun and easy to learn, and offers composers, researchers, and performers a powerful programming tool for building and experimenting with complex audio synthesis/analysis programs, and real-time interactive control.
I believe the end result can be converted into MIDI, which can then be converted into a score or sheet notation.
I don't know if this is what you're looking for. Hope this helps!
EDIT
After thinking about this a little longer, I think what you can possibly do (and this sounds a bit crazy) is write code that generates ChucK code. So define a set of rules for your music/score generation and then use that to create valid ChucK code. After you run the ChucK code, you can get a MIDI file which you can then convert into score/sheet-music.