Well, I went ahead and did it. Noodled on the topic for a while, and finally figured out a fairly elegant solution. I had envisioned some sort of ratchet strap system, but when I was on Pelican buying my Dansk "dad" muffler to replace my M&K, I saw an ad for simpson harnesses. Initially thought I could jerry-rig a six-point harness, but they had lap belts for vintage racing.
The seat is a Cybex Aton 2. One of the best (and smallest) car seats on the market. Crush foam, secondary crush pieces on the frame, all sorts of good stuff. As a bonus, it adapts to my stroller (Bugaboo Cameleon).
I pulled the rear passenger-side seatback (an easy affair with four screws). This gave me a few inches of rearward travel. I pulled the bolts out of the seat belt anchors on either side of the seat pan, and installed race-grade harness anchors in their place. I then put two simpson lap belts on the anchors. One goes over the car seat, and one goes around it. This is a configuration known as the "Euro" mount. It dispenses with the base.
These harnesses and bolts are built to hold a 200-lb man in place in a full race-speed crash. With this system in place, I can rock the whole car using the car seat. I've trusted my life and my wife's to this car for a decade; I trust my child's to it now.
By the way, Jack was born on Tuesday. He says hello.
