I did not have a lot of time to find a solution for this so I used a workaround maybe that can work for your case as well.
So firstly my case was such that I needed to have an entries field type that only the admin can edit for each user. It was NOT important in my case for the user (For whom the field is being edited) to see which entries are attached to his field in the backend control panel. I could show it to them in the front end. Hence what I did was I modified the entries fieldtype form template (Which is located at craft/app/templates/_includes/forms/elementSelect.html) and added two if statements. Firstly I added an if statement to check the name of the field and then another if to show or hide the add entry button. Hence if my entry name is userSubscriptions then the html looked as follows:
{% if name == 'userSubscriptions' %}
{% if currentUser == 'admin' %}
<div class="btn add icon dashed">{{ addButtonLabel }}</div>
{% endif %}
{% else %}
<div class="btn add icon dashed">{{ addButtonLabel }}</div>
{% endif %}
But obviously this is not the best way of doing it. Its just a temporary solution if you really need to do it fast. Im not sure if this could work with your fieldtype as well but thats my take on it. I will keep trying to find the correct way of doing this and if I manage to do it Ill let you know and if you manage to do it the other way then let me know how you did it. Thanks