This is a followup of this post.
Right now I've managed to add a new setting in the form general settings (so this is not a new field type, but a form property).
I've tried to do this following the example provided for the 'gform_field_standard_settings' hook.
After struggling for a while and trying to understand how standard form's settings work, I've realized that this hook implementation must be different, even though the docs says "This hook functions identically to the gform_field_standard_settings hook.".
Anyway, what I think I've understood is that rather than using the SetFieldProperty() javascript function, I must use UpdateFormProperty().
However, the value is not stored.
Debugging this function I see that it tries to set the html content of "#gform_" + name (where "name" is the property name I've created), but this element doesn't exists.
Here's a sample: http://pastie.org/5412888
I'm really lost: I've spent the last 4 hours trying to sort out what I'm doing wrong and the official documentation is not really helping.