I've done some quick drawings based on Ozzman's dimensions. This rack is designed to be attached semi-permanently. Rather than prying the tow hook covers off your bumper each time you wanted to mount your rack, you would install a short adapter that, when threaded in, would be flush with the bumper cover. Here's an exploded view of the two parts for each side:
Having threaded in the adaptor (top), you would then apply a 1-1/8" hole saw to the tow hook cover, making a hole for the flush-fitting adaptor. Then you'd install the drilled tow hook cover over the adaptor, resulting in a clean finish (or a sano, if you're a hot rod guy). The idea is that you could leave your car set up like this (with the flush adaptors in place) all the time. Weak Loktite would secure the adaptor without preventing its removal in the future.
When it comes time to put a bike on the back of your car, you just get the adaptor extensions (bottom) out of your trunk and thread them into the flush adaptor threads. Here's an image of the adaptor and extension threaded together:
The female thread on the adaptor is the same 16x1.5mm thread on the tow hook...if you needed to use your tow hook, you could just thread it directly into the adaptor. The way I spec'd it out, all parts are made of 304 stainless, so they could easily handle the load from the tow hook. (To do this right, I'd need to add a female chamfer to the adaptor and a male chamfer to the extension so that the tow hook chamfer has something to engage. I haven't taken the time to do this yet).
With the adaptor made from stainless rod and the extension made primarily from stainless 1-1/8" tubing, the entire assembly weighs 950 grams per side (almost exactly two pounds). That means your whole rack weighs four pounds, which is pretty good (IMHO).
There are still some issues with this design, but I'm reasonably happy with it as a first draft. If there's enough interest, I can create some formal mechanical drawings of the final design and make them available to list members.
Cheers,
Jason