That's an interesting point, and not something I recall ever seeing handled in an MMO. I admit that the "sim all the things!" designer in me wants the stirrups to be manually adjustable now, with penalties to the noise being made, your relationship with your horse and your ability to command your mount when you ride, if you don't adjust them properly.
I'm not saying we're going to do that, mind you, but I am definitely feeling the impulse to do so. ;)