A good number of Dudley Dix's designs are spec'd to have the settees function as integral water tanks. Especially the Didi 38' "Black Cat" & it's spin offs Didi 38 radius chine plywood sailboat
With quite a good number of them having been built. So you might put in a polite query with him, as well as with some of the folks who've built the boats. He's a very easy gent to interact with.
Also, most of his boats with integral tanks, Didi38's anyway, have been home built. And I'm sure that a number of these builders would be happy to share their experiences. Especially as many of them even having build blogs linked to Dudley Dix's website.
BTW, I gather that you're well aware of; food
safe epoxies, post-curing, & steam cleaning
, etc. It just seems worth mentioning, to confirm that you know of such tools & practices.