How to scribe a circle in the ground...without access to the middle point?
If the ground is perfectly level, then you don't need access to the middle of the circle, you just some point directly above it, and then a longer string traces the shape of a cone as you draw your circle.
As you said that you need more precision than having a person just stand on the rocks and hold the string, you will need build some sort of structure to hold the end of your string stable above the rocks.
You could:

Use three stiff pieces of wood or pipe to make a tripod that goes over / around the rocks.  Tie a loop on one end of your string so it fits around where the three pieces join, and then mark the string where it comes to one edge of your circle

If you're going over something that's relatively long, you may want to build two supports on either side, to hold a pole with the string tied to the middle of the pole.  But in this case, you'll have some problems where the supports interfere with the string, unless your center pole is larger than the diameter of the circle you're attempting to draw.


In the second case, the pole needs to be really stiff, and the string securely fastened to the pole, or you're not going to end up with a perfect circle.