By "mask" do you mean you want to hide the actual text in the field, or lock the field to prevent changes?
You could use what is referred to as a password field where the characters are not displayed. If you are looking to prevent changes, you could use a digital signature where the field is associated to the signature, once signed, the field is "locked" to prevent any changes, also, if if the data is somehow changed, the signature will be invalidated.
A Rights Management policy is applied to a document as a whole, it cannot be applied to a "section" of a document.
Regards
Steve