Mercanaries

Has there been any official word on how Mercs work? Because the Medival mercanires were very sad in my opinon, leaving it impossible to replicate the armies of the Byzantines and other mercanary heavy factions. It would be incredible if we could recruit mercanary armies (like that famous Catalanian Company that was recruited by the Byzantines.) Or we could demand troops from our vassals. For example as the Parthians, after forcing the Selcuids into being a vassal kingdom, would it be possible to demand X amount of troops per year?
Just my thoughts, and any response, official or otherwise would be apreciated.

The only thing I have heard on mercanaries was when captain fishpants was telling us about his campaign, and how he finaly found Cretan Archer mercanaries after much long and hard searching. So either good mercanaries are going to be hard to find, or perhaps they are extremely region specific.