Adam, sorry I couldn't find what you need. I have done the same search in the past, and failed to find anything. At least is demonstrated that it is not easy to find :-)
I'd suggest trying those longer wire single-row headers. I have only tried stacking two single row headers once, but I had poor experience with the 0.1" female sockets I had to hand. They were too tight, and bent outwards, but as you can adjust the exact position of holes on your board you could fix that. In my case, the board was ready made with quite tight holes, and there wasn't much I could do.
I think some of the problem might simply have been to do with the thickness of the plastic walls of the socket, and there may well be some which stack nicely.
For example several companies including digikey (http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1442673) sell wire-wrap sockets which might give an alternative to try out.
Another technique, which is a bit odd looking is very long pin single-in-line sockets (SIL's). They are designed to carry IC's, so the 'socket' part is very shallow, but it is better than nothing :-)
The ones I've used do stack.
A friend found some a couple of years ago, I'll ask him if he can find them again.
Best of luck, and please tell us if you find something.
