Someone may have already mentioned this, but if you're looking for stems that long to go with short, shallow bars, have you considered Nitto steel or alloy bars with a slightly higher front end? The drop/reach on them is quite a lot compared to short, shallow road bars, but I have a set of steel B123s with a 6 degree, 130mm stem and run a couple of extra spacers under the front end. I've never, ever, detected the slightest bit of flex from them, (ymmv) but I can feel flex in most alloy road bars (3t, Zipp, Deda). Something to consider, even though it's not quite the answer to the question you're looking for. I think, if you can find the SB models, they are the ones with a 31.8 clamp, see the link
NEW NITTO
about halfway down the page.