While I cannot comment on Priority Pass policies, I know that many airlines have specific prohibitions against travelers on non-revenue tickets utilising lounges. This holds true whether the staff have purchased access to the lounge, hold membership to the lounge or are accessing the lounge at carrier expense. In those cases, if you are traveling on a non-revenue ticket, you are not permitted to use any lounge. This is true in case of all tickets issued by any airline under the ZED agreement, which explicitly provides that "use of special service facilities such as airport lounges... (is) not permitted".

The lounge (especially if it is a third party operated lounge) does not care or enforce this policy, but the airline authorising the rebated travel reserves the right to do so. Violations could potentially result in removal of benefits or other disciplinary action being taken.