As far as Ive been able to figure out with my crappy coding skills, there are currently no supported code for having "fabric" such as flags, banners or superman-capes for that matter, implemented into the game other than as a static flat piece that does not move. So scripting changeable cloaks that pretty much change to another cloak when you mount up [I see no way around it], would be hard to say at least.
The only place in the mount and blade game where you could achieve this is on the world map where both banners and fabric can be animated to work with stuff like wind, movement, etc. On the actual playerfields like cities, battlegrounds and so on, we wont have realistic looking cloaks for a while unless someone does some really hard, hard work with the physics engine, methinks...
One other problem is off course that there really is no equipment "slot" for this so it has to be welded to either the chest armor or helmet....