Prepopulate + Field Extra Widgets?

Hi

I can use Prepopulate to set the value of group_content_access (aka Group Content Visibility) when creating a new group content node. I would like this field to not show on the add or edit form, and instead just get its value from what Prepopulate fetches from the url. I tried using the hidden and readonly widgets provided by Field Extra Widgets but couldn't get the Prepopulate value to save.

Any suggestions as to how to meet this need?

Thanks