I've used 27.2 aluminum, titanium and Easton carbon seat posts without any issues on our tandems. The Easton EC90 was a fairly robust CF seat post but, like all captain's seat posts, success or failure is not a level playing field. I don't have a lot of exposed seat post mast on our road tandems and given our riding positions Debbie's stoker stem clamps on at the bottom of the seat post just above the seat tube clamp, see photo below. The same was true on our other road tandems.
Debbie is also a very slight stoker of 62" in the 110-120 lbs range who doesn't input much in the way of loads on her handlebars and I'm also a very lightweight rider. So, our success or failure would not be as useful as a benchmark vs. Merlin and his stoker who are both larger riders, or with someone whose stoker has to have the stem attached at the top of the seat post, just under the saddle clamp. Just something to keep in mind when getting feedback on what works and what doesn't.
Now, all that said, I decided to use Thomson's aluminum Masterpiece seat posts on our Calfee build back in 2008 because after comparing them with all of the other alternatives to include the composite posts that were available at the time, the Thomson Masterpiece were about as light as the composite alternatives and less expensive. Moreover, the Thomson seat posts are notoriously robust. So, as you evaluate your alternatives be sure to look at the cost benefit of the various different materials used for some of those component decisions, you might be surprised by what you find.