There are very few substitutes for the 'ole post 'n hole system. Nothing else can hold the body onto the car during a crash like the posts can (they are tough but also flex). Magnets and other wacky inventions have been tried but they do not work well (especially when you hit something).
Velcro could work, but you would have to have a sufficient contact patch on each corner to keep the body in place, this would more than likely require some custom mounts near to the locations of the posts, adding weight and generally getting in the way. If you need a car which does not have any posts sticking through it, then velcro is probably your best option, if not then dont bother,it is unlikely to worth it in the end.