Is there a SOP (either native, or Labs or qLib) that will take a rectangular poly, floating in space at some strange angle, and align the sides of the poly to major axes (whichever)? I can compute the poly's normal and use it with "dihedral" in vex, but I wonder if there's a native SOP just for this.

You can use Bound SOP (Rectangle) and Extract Transform SOP to do this:

Cool, thank you.