Hello, long time reader, first time poster here.
I searched around on the forums for a while and didn't find anything like this, anyway: yesterday i tried how to mount at Dalaran bank and finally found a route that seems to works perfectly all the time.
First, start outside of Dalaran, at the Violet Citadel.
Start flying towards the citadel and land on the roof, since you should get the "no flight area" debuff and remount once it's worn off.
Now look to Dalaran bank and start flying there, once you're above stairs fly straight down to them.
If all goes well you should be safely at the bank without dismounting!
Have fun and enjoy!