Triadic,
I agree mate. In fact I faced that very scenario on more than one occasion and there is nothing in the legislation or guidance that says how it should be dealt with. Common sense says.... no intent to act illegally so help said pilot out to get aircraft parked and sort out the access matter later. You wouldn't just let them have the gate code but you can use discretion such as escorting to and from the aircraft etc. Probably not 100% in compliance with legislation but I guarantee if you call the Dept and ask for advice you will be treated to the sound of crickets. At the end of the day, it is quite a rare occurrence and depending on the airport each ARO / manager will deal with it differently.
Its one of the may situations that were not fully thought through and because the occurrence is so rare, it is unlikely to gain any momentum. Remember the regulator's office is full of people who in 90% or more of cases have never worked a day in this industry in their lives.