This is a simple trick to get on top of any Arathi Basin flag. For this trick I used the Brown War Bear mount because of it's size but I would think any large-size mount would work just fine.
I will use the Blacksmith flag to demonstrate. Mount up and position yourself between the BS flag and the actual BS building. The idea here is that you want to wedge yourself between the two flags hanging on either side of the BS flag post. Mount up, ride straight at the flag and give a long, running jump while aiming yourself such that the nose of your mount goes right between the two flags hanging on either side of the flag post. If successful, you will become stuck there. Here is a screenshot of a successful jump:
Once in that position, just dismount and use some old-school wall-jumping (jump and tap forward) to get to the top of the flag:
Enjoy!
I did a search on this and there were several threads on Arathi Basin but nothing I could find about getting on the flags. If this is a repost, then I apologize ahead of time.