Apologies if this has been mentioned before, or is frowned on by rock art enthusiasts, but the takuhon* technique may be of interest for recording rock art when other recording techniques are not very suitable.

Takuhon is similar to the Western technique of taking rubbings from stone or brass - though it differs from the Western technique in so much as it doesn't involve a rubbing action at all. Takuhon was first used in China around 600ce to take impressions of inscriptions that had been cut in to stone. The technique involves using slightly moistened Chinese or Japanese paper which is tamped over the inscription with a cloth pad so that the paper is gently pushed into the incised areas. The paper is then lightly inked with a second cloth pad and the paper then peeled away. The result is a crisp impression which, as far as I know, is achieved with no damage to the original artefact.

There's a related technique/art form called gyotaku** (fish print). Gyotaku is similar to takuhon and can reproduce a finely detailed print of the original.

* http://sunsite.berkeley.edu/Rubbings/rubbings.html
** http://www.whitemouseflyfishing.com/gyotaku.html