Hello !
I just found a really simple method to access PTR if you already have beta on your account.
1) Connect to your bnet account
2) Choose your beta account, then choose "Character copy (beta realms)"
3) In the url, change "beta-select" with "ptr-select"
4) You can now copy your characters to PTR realms, and when you go back to your Account summary, you now have a new account (WOW4 for me) next to your beta account, and full access to both PTR and beta.
I know this is not a hardcore exploit but it's the easiest method I've found so far.
Enjoy