Install ground rod - middle of the winter!
Just wanted to post the success.
Ended up renting an SDS max hammer drill and ground rod driving bit and also a 5/8" masonry bit. Predrilled an 18" deep hole for each rod, then set the rod in, and pounded in using the hammer drill with the rod driving bit. It went straight down, easily, no problems at all. Left about 12" above grade, attached #6 copper to both rods (continuous run) and fed into the house.
Thanks for all the advice!
I'll wait for inspection for sure, probably even wait till spring, then drive the rods the rest of the way down and trench out a little for the #6 wire.