I have to agree with the others, don't try to add any anchors. Just use the lap belt--if you do it right, its just as safe as LATCH and probably safer than anything you can rig up.
My three year old is on the driver side in a LATCH connected Nautilus, and the baby is on the passenger side in a Snug Ride. I attached the base using the seat belt, and I can just leave it alone since I drop in the carrier. I used the little metal bracket the carseat came with, and remember to pull the seat belt all the way out to lock the retractor.