No progress report, but some notes. Mountain bike tires usually have their max width on the casing somewhere but they also have a shoulder at the edge of the knobs and their cross section is sort of lightbulb-shaped. This frame does not have any cross members to worry about at the OD of the tire and of course the stays get further apart as they go back, so this is the place to worry about clearance. Surly and Schwalbe both helpfully have dimensions for their tires at this location. The shoulder for both 27.5+ Dirt Wizard and for various Schwalbe 29er x 2.3 (60-622) is around 345mm from hub center. For 29er tires the shoulder is up to about 62mm and for 27.5+ it's 75mm or more. And then of course it needs some clearance - which is why most of those 27.5 plus tires will fit in a 29er frame with some close clearances. There are some tires listed as 27.5x3 that are really 3" (both WTB and Schwalbe have both 2.8" and 3" tires). There are some that are listed at 3" but are really more like 27.5x2.8. The Dirt Wizard seems to fall into the latter family.

Looking at my stays, I'm thinking the limiting factor will be not the curved chain stays but the straight seat stays. They sit a bit wider by the hub because the upper part is bolted on the outside of the dropout, but up at the seat tube they go together pretty closely. They're about 1.5" apart at the top and 6" apart at the hub. They might be a little bent at the nodes. Will it be enough, even for the 29er?