How to theme a field formatter?
What I wanted was actually to theme the registration field directly from the module, by implementing zss_field__field_registration($variables).

So, if you have a similar problem, you need to proceed in 2 steps:


create a custom field formatter for your registration fields (described above). This will allow to display the form even when the current user has already registered.
theme the registration field from your module so as to display it according to the current user and available seats.