I used to do that a lot on big carparks when I was frustrated by the lack of wind..you really don't need much wind..an old 4.5m and very ligth wind is perfect to do lots of tricks, backwind etc...plus if you have good skate skills you could even do some aerial gybe/willy skipper kind of move ( I can't I am a crap skateboarder).
As for fixing a rig onto a skate nothing is much easier, just drill a little hole in the skateboard for the screw thread to go through the board and the square nut to be just behind the front truck. This way when you screw the mast foot the square nut is blocked (from rotating) by the truck . Easy and you only have a little hole in the board!

Fish is right , I saw some cheap longboard skate with big wheel in Kmart/BigW toy area .. would do perfect for grass