Is there a SOP trick that would remove the "diagonal" edges from triangle-divided quads in a poly object? I.e. in the pic below, I have what's on the left and I want what's on the right. I don't want to actually quadrangulate, nor do I care if some quads will become convex, I just want to remove the "diagonal" edges. I could write a point wrangle to loop through outgoing edges and test angles, but I don't want to reinvent the wheel, if it exists.

Peek inside the labs quadrangulate and look at the 'group_long_edges' wrangle, it has a pretty good setup to do what you're asking. You can steal that code, pass the group it generates to a dissolve sop, and skip the labs hda's step to try and refine the ngons.