At my airport I work for Pax services. We sometimes have crew/engineers from airliners that are not based at my airport; therefore security will not allow them to go through the Validation Point with their own ID. So all we do is write out a manual boarding card and send them through the passenger route. Once they are through security they can then go over to a gate and ring our operations who will collect them from there.
So security would NOT allow them through on an ID which was issued by another Airport and approved by the DfT, but all I had was write their name on a manual boarding card and then they could just walk through the passenger security.
Ingenious! Unfortunately if your engineer is caught s/he will be arrested. Not worth it really is it. And you could lose your job to boot. The rules are there for a reason. Why is it people see them as a challenge rather than something to adhere to? They may seem crazy, we may not like them, but they're LAW.