As long as she is on an uneven surface - doesn't have to be "rocks" per se - you can place rocks underneath her feet and cast the appropriate shadows on them to make it believable.

Forgot to ask about whether or not she was going to have a wet-down for the shoot - occurred to me after I posted. Awesome - way to think ahead.

Yeah - I did a shot of an Excalibur sword for a catalog company that was in rocks in a stream with a small waterfall behind it assembled from about 6-8 photos including the sword. Various rocks were resized, shadows adjusted, moss painted on and overlaid with other shots of moss, etc.

I think they got them from www.photos.com because they (the catalog company) have a subscription to that site.

Good luck and have fun with this!
-Lew ;-)