Hello. I have discovered this pre-3.2 bit it should still work.
What you need to do:
1. Scam some gold that will you get taken away if you get banned.
2. Buy as many mounts as you can
3. Wait ^.^
4. After the ban log on and see what's been taken away. All the mounts should be taken away but you're achievements stay the same.
Now you should be able to do add the mounts again and it will add to your mount total.
Screenshots: