Shop vac cut off the end of a rubber shop vac fitting, and see if you can get a good enough seal to just pull it up. You could just apply some urethane caulk to the end of your shop vac straight pipe, and set it on the rock for a couple hours to get a very good seal, and then suction it out. I know my shop vac will lift 3-inch diameter river stones, because I use it to excavate a gravel sump for Radon suction ports in slabs, through the 4-3/8-inch holes I drill in the slabs.
Failing that talk to a septic pumper, and see if they can suction it out for you. Their truck have considerably more suction than a shop vac, and move a lot more air. Might cost a bit, but would still be less than having someone dig down 10-ft, break into the line, to get the rock, and put it back together.