@Nik I believe in client script you need to get the field and then set

.isDisabled = true;

a bit like the below.

var currentRecordField = context.currentRecord.getField({fieldId: 'myfieldid'});
currentRecordField.isDisabled = true;

Tried this already, but didn't seem to work on LineItem

hmmmm that is what i am using on a line item on IR