Just to follow up, I've had very little response outside of this forum, but did get some advice from people in the U.K. Why the eBay uk listings never showed up in my google searches, I have no idea, but they've been quite helpful. I'm so glad I took the time to ask before proceeding (before losing my shirt!) with his offer.
There are a number of twin Weber sets for sale on eBay uk, but very little in the way of 40 DCOE 31's. Everything I've seen so far has been thoroughly ultrasonically-cleaned, chemically-dipped, and/or soda-blasted, and the steel parts wire brushed, but the close-ups still show considerable fine pitting, so they were significantly corroded to begin with. (One pair actually had the exact model numbers "cleaned" off, possibly just over-zealous refinishing, but...) The prices are firmly in the ÂŁ500-700 range for a pair, which is right in line with the US$400 apiece DrEntropy suggested, and mine are as-removed, original and unmolested, which is what I'd prefer myself if I were buying used carbs. I think I'm going to put them up at that range and just use the cash to fund my engine project.
Thanks to all who clocked in to prevent me making a costly mistake!