Sup, another wall clipping tutorial. (This was done as a dwarf on a rug, this is known to have a smaller clipping box than other race/mount combinations, but I also tried with the blue drake which was a little harder to get through)
First, go to this position, near ICC.
Fly forward and down into the crevice, wiggle a little and you'll clip through. (alternatives are to the left and right of this one) Fly upwards to this point.
Behind the pillar in the above screen, there is a small raised area with a square on it, fly to it. When you get there, you will notice a barrier, just fly upwards until about this point.
From there, you can fly anywhere you want, including wintergrasp, as seen in this shot:
If you cannot fit into any of the above spots for whatever reason, I found this other spot that has worked for every combination I could try.
You can also just walk through this one. (careful, long way down)
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You can fly just barely into the icecrown hall leading to the raid portal so that people can target and attack you, but you remain on your flying mount, while still retaining the ability to fly back through. You'll find you can do this in a lot of places.
Using that method you should still be able to fly around with your mount.