Well, you're right aboput that BE! I do plan to go through the marginally beneficial HDD water cooler, and even though the MOSFET cooler seems complicated, and the benefits dubious, I do believe that that's one worth the effort.
But I certainly wouldn't go around telling people about running RAM at more than 2.9v without a fair warning!

That aside, I can see water cooling ram at above 3.5 helping to save the RAM, for sure, but there'll be other support chips that'll be running way outside spec. If I was running RAM above 3.5 (which I'd never do), I'd keep a close eye on the northbridge temp, and the power supply chip(s) to the ram, along with anything else that's related, and running hot.
Could be fun, but too risky for my taste.