I try to ignore shimmy being blamed on frame stiffness, but single oversize tubing generally the same torsional stiffness as standard tubing. Because of wall thickness. But if a touring frame shimmies with a load on the back, put more of the load on the front. Many bikes will shimmy when rear-loaded. Some will do it when you sit back on the saddle. I know people in the U.K. like to ignore the French, but French cyclotourists usually had front panniers from way back. And American companies started pushing front panniers in the '70s. It's like there may be something to it.
Having said that, Jan's "mule" has an oversize downtube. And he's the biggest advocate for flexy tubes in all of humanity.