Fields to be made non-editable in Resignation Approval Page

Summary:

We have requirement in our current project, when employee applies for resignation it will go for Line manager Approval.

When the LM receives the notification LM is required to edit and provide input for only one custom field. All the other fields on the page should not be editable for the LM to update. When we are trying to use EL expression to control the access we are seeing that its having impact in the Resignation Submission page for employee, but we need to restriction only for the approves.

Has anyone come across such a requirement and how was this handled?