Interesting issue. In 2.3 of Formulize, I think you're right, you could not have an element editable to only one group but visible to more than one. Visible and editable is the same thing in Formulize 2.3.
However, if you go tohttp://www.freeformsolutions.ca/formulize and click the link to get the files from the FSOSS workshop, and download the XOOPS files from there, the modules folder will contain a more up to date copy of Formulize. You can follow the instructions from the 2.3 readme as far as installation and upgrades go.
In the newer version, there is an option to make a form element disabled for certain groups. That should suit your needs.
Good luck,
--Julian
Formulize - custom registration forms, ad hoc forms and reports