Found this by accident.
Basically:
1. Set hearth in an inn where u wont be dismounted if you are on a mount (I.e. StV, moonglade)
2. Go to northrend, and get on ur flyer.
3. Que for a dungeon while on the mount in the air.
4. Get in dungeon then hearth out.
5. you will be on ur mount where u set ur hearth in Azeroth.
Note: you can even get on a boat to another continent and still stay on your mount. You can fly anywhere excpt Hyjal cause u get ported out XD.
I did this on a druid so my hearth was in Northrend and i just TPed to moonglade so it is easier for druids. Also i was in the dungeon Mags lair.
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