Hi Nate.
In libGDX runtime MeshAttachment not implement rotate. In RegionAttachment i see next code
if (region instanceof AtlasRegion && ((AtlasRegion)region).rotate) {
but in MeshAttachment do not see any rotate modifications
vs 
I think need improve runtime or not rotate mash in export, rotate only regular slot. I have many animations and only 5%-10% use mesh.
Regards,
Sergei.