I thought this was a great idea. I have a church with a Huge parking lot less than a mile from my house. So I set up a mini track just to work on getting off the bike. This parking lot was very nice and clean... no sand or other debris, very few tar snakes as well. Unfortunately the surface had a sealer that was much slicker than I thought it would be and on my second lap I pushed the front, dropped the bike and walked away with about $1000 in damage + a sore back. I just feel very lucky that I went down where I did... the next corner had a concrete planter that the bike and I would have hit and with no medic around, and no one else to see that I'm in the parking lot that could have been bad.
Most parking lots are private property so be aware that the owner could call the cops on you unless you make an arrangement with them ahead of time. If you’re going to play in a parking lot, remember your tires most likely are not warm enough, the surface is meant to park cars, not race bikes so could be slick. Also make sure you are with someone who can call for help if required. Someone warned me of this, I wish I had listened, but you live and learn.