On the Carradice site, I see this SQR system, and that appears to hold a bag a couple of inches higher than attaching to the saddle would. But, there's no support betlow the bag, either, so you may get some extra droop, and I'm not sure what the net effect would be.
A friend of mine had a Carradice bag that was noticeably shallower, but when I was looking through the website a year or two ago, I couldn't match it up with a current model. See the second photo there, next to a "Junior" model, I believe. I was thinking the rack she used with it was shallower as well, but in looking at it, it is very similar geometry to the others. (I think the "Sport" rack is a little shallower than the "Expedition" rack, for that matter.)
The Carradice racks are just a bent rod inserted into the clamp part, and if you can locate some rod of that size, it wouldn't be too difficult to bend it to shape- the challenge would be getting it symmetrical and making it look pretty.