Glad you brought this up! I have made a Byzantine mod that gives the empire gunpowder, better spearmen, bowmen, and ships. Check it out!
http://files.filefront.com/ByzEnh12e.../fileinfo.html
The additions include as of the latest version:
Touflaki Scutatoi (musketeers)
Arab Gunners (gun cavalry)
Bombardizos (Byzantine Naffatun)
Scutatoi Pikemen
Kataphractoi Archers
Klibanaphoros (heavier cavalry)
Scutatoi (better spearmen)
Tzangratai (crossbowmen)
Hand Gunners
Arquebusiers
Galleas
Cannons (Ribault, Monster Ribault, Mortar, Culverin, Monster Bombard)
Added Byzantine Infantry to castles, which I renamed Themata Spatharion
Pselloiphoroi (late period heavy infantry based on the Zwei Hander)
Please read the readme for install!