Having over 12 years of experience on wireless networks, both short and long haul, I can tell you that shooting the signal from the parking lot won't be an easy feat and even if you achieve something, it won't be worth the effort. Regardless of how directional the antenna can be, the frequencies in use for WIFI signals hardly ever transverse walls and survive to tell the tale. Yes, some of it would make it thru, but you would loose so much of it that it would hardly have any effect. You would need an antenna that is far more noticeable than you would like. At best, you can try to buy a high power amplifier (worth hundred if not thousands of dollars) but then I would not advise you seat close to it for too long.
Harder than getting your signal thru the wall is getting the return signal from the devices. In order to save power and make your battery last longer, many device manufacturers limit the transmit power of their built in wifi chips.
I think another possibility would be to stuff the Piratebox with a high gain antenna or a moderate amp in a lunch cooler. Leave it under a chair and walk away. Come pick it up later, or if you can afford it, leave it behind for them to ponder about the satanic plots behind such contraption. Put some scary stickers on it.