At this point the only way to do this is as follows.
1. Set up read only sharing as default on the Page object. This means users will not be able to change pages they don’t have rights to.
2. Make a page that only has the required field on it. (one field editor, one field). It should be owned in such a way that the end user will not be able to share edit rights with this page.
3. Make a page with the rest of the fields in it. This page should be owned in such a way that the end user will be able to have edit rights. Maybe use criteria based sharing rules.
4. Use the Page include component to bring in the “required field page” into the editable page.
This may seem like a hack, but it is supported.
