I don't think you can reliably piggyback anymore. I come and go at all hours and there is always someone at the booth now. If you have any friends at Republic, USAir, or Air Wis. that are PHL based but don't use their pass, you could use theirs. I've never had my ID checked to make sure it is mine. It had better be a good friend though because it's a major PIA to get one. Also, at Air Wis, some PHL based pilots that don't use their parking pass get compensated for parking elsewhere, and are then ineligible to get one in PHL (they are eligible to get the equivalent amount that it costs the company to get them a PHL pass).
If that doesn't work, there are several private lots around the airport that cost $7-$9/day. Good luck.