Show a list of all forms
Well I ended up using Hook_form_alter () to add every form that was being called (and was not on my form list yet) to a form list. 

Then I wrote an admin interface that enables the administrator to select what forms are still active/should be listed. Next to this I implemented a REST service for the Services module to GET the list of forms.

The fact that the hook_form_alter () has to preform a DB request to see if the form is on the list everytime a form is viewed is some what performance heavy, but since there is no help, this is the best I can think of. If anyone has interest in the code, drop me a line, ofcourse better ideas are welcome;-)