I found an alternative solution for the fields on the WOACTIVITY in the Tasks for Work Order table in the Plans tab. I implemented an Automation Script with an Attribute Launch Point for the field(s) being targeted (launching on Initialize Value event). In the Automation Script, I assess the MBO internal status value:
mbo.getInternalStatus()
If the internal status value is not equal to 'WAPPR' then I call:
mbo.setFieldFlag('<fieldname>', MboConstants.READONLY, True)