Sorry but I find the idea totally wrong. If you should be in the circumstance Megadread initially suggested and you're desperate to see it, make arrangements to go back in better light or with water.

There's a news item on site now about someone defacing Uffington White Horse, and on this thread there's serious consideration to pissing on an ancient piece of rock art? Someone got arrested for urinating on a war memorial last year, anyone here disagree with that?

Besides, my piss would dissolve the whole stone....