I question whether the issue is actually the rear main seal or if there is a defect in the block. Back in early 2012 my dad ordered a brand new Charger with the 5.7. When he brought it by on his way home from the dealer, I noticed a drip in my driveway. We got under it, and saw oil in pretty much the same spot as you guys. My first thought was the rear main seal due to the location.
We brought it in, and they said the seal looked fine. A rep from corporate ended up coming out and believed it to be a flaw in the casting. They replaced the engine and sent the old one to Auburn Hills for analysis.