This will seem an odd reason for a first post but I've been keeping an eye on this mod for some months. It promises (when complete) to be just about my favorite thing to come from the TW franchise, given my particular historical predilections.
A band that I think would suit the subject matter perfectly is one from Bulgaria named Irfan. They dwell at the musical intersection of Europe and the Middle East and there are lots of religious overtones, not to mention a distinctly "ancient" sound. For example, and I apologize if there's a better way to provide a link:
Potential battle music
Good for campaign map
I'm not sure what obligations a third-party, not-for-profit modding team would have as far as obtaining clearance, but my notion is that they would be glad for the exposure.
If this thread isn't redundant, it would be a good place for others to air their suggestions...