How to create custom access permissions for entity form
The reasons the dpm()'s in your hook are not showing is that this hook function is not being called when you access the form as an administrator (administrators bypass all access restrictions). I tested this for authenticated users, and it does work. The hook is called as expected. (To test it yourself be sure to allow devel permission to the authenticated user so that you can see the output of the dpm).  The hook is being called when a user accesses the form.