Hmm... The main thing I'm not sure about with the current system is that you can instantly change your entire mounted force, customizing it against archery or mounted units. I don't want to make one available in desert and the other in plains, because I don't want them to have plains at all... If I keep it the way it is, it would either become a spell with a significant turn delay, or a one time promotion.
Edit: Read what you wrote again, think I could actually introduce a Camel Stable building. Units in cities with the Camel Stable can become Camel Riders, those in cities with a normal stable can become Horse Riders... Would be perfect, especially if I can make a building mutually exclusive with another. Not sure if I can or not, have to check.
Edit 2: Sadly, there is no building exclude tag. I know I saw that Jean Elcard had a mod that did that, but it was for vanilla BTS.
Edit the Third: Decided to go with the Camel Stable. For now, both are able to be built in a city, so while the promotions can still both be taken at the same place, it takes a much higher hammer investment. Looking for a way to block them.