Custom fields should support read-only for public portal with edit for internal portal

We have an idea review process for one of our Ideas portals that uses several custom fields. Two of these fields are used for the outcome (go/no-go and reason for decision). These fields can only be updated in the internal portal by users with paid seats.

We'd like to share these two fields with public portal users rather than duplicate the info in comments. In order to do this, we need to make the custom fields on our custom layouts read only.

The ask: the read-only attribute has one value on public portals and a different value on internal portal views. Read-only can be set one way for paid seats and another way for everyone else.