Progress!
Left is standard curve-to-mesh with line primitive profile pointing in X+, right is WIP Z-Up-Even-Thickness experiment only offsetting a single point in X+. The “secret-sauce” is that the “scaling-fudge-factor” for some point projected along the normal (curve point’s local X-axis) is 1/sqrt(ang/pi), where ang is the curve-angle (in radians) for that point. (Not doing the radius method - going the route of custom projection like in my Z-Up-Curve-To-Mesh)
More than 14 hours of effort later and still too much of a WIP mess to share the .blend cos I went down the torturous rabbit hole of supporting multiple curves-per-object were any of those curves could be cyclic curves. Lots of cleanup still need to happen and would like to wrap it in a custom curve-to-mesh node-group similar to my Z-Up-Curve-To-Mesh. Just wanted to share the breakthrough of just getting it to work.
This is standard Curve-To-Mesh
…But here you see I’m doing a custom z-stepping of the curve which may not be what others are after but is something I’m working toward…
It was waaaaaaaay more work than I anticipated 
but I’ve needed something like this for ever so hopefully it’ll be worth it. Will post the final result to tips&tricks (hopefully sometime before Monday).