Read-only custom field? Also a rights problem?

RT 3.8.7

I’m looking to establish a read-only custom field, writable
only by $RT::SystemUser (in a scrip).

In trying to do so, I have made the rights to this custom field
’SeeCustomField’ by some people, but nobody has ModifyCustomField

That is:

 there are no defined user rights for the field.

 there is only SeeCustomField rights for group

I am still able to modify+save the field though, so clearly I
am doing something wrong here. Any help would be appreciated.