Law 5 wouldn't mean a thing. The laws of the land will trump the rules of a game every time. You'd be very naive to think that just because the rules of your chosen game say you can't be sued that you can't be sued.
I think there has been a couple of attempts against the referee (say, when a player has broken their leg by catching it in a divot) in the UK but nothing successful.
While there has been no successful case, to my knowledge, you want to do everything you can to minimise the potential for it even being worthwhile of getting to caught. Not a lawyer, but I'd imagine there would be many ways to get around Law 5 - I'd imagine simply arguing that the referee hasn't fulfilled his duty (say, ground check) would be the easiest way.
But also, if the match isn't sanctioned and it does get to court, would the FA protect you? Or would you be on your own with the lawyers?