parametric plates with curves

I have a project with a lot of plates that have multiple arcs, curves and tangents.

Create plates using polygon shape works really well for creating the plates.

But if you try to modify a side or a vertex using the grips the points that were created as tangents are no longer tangent points.

It is usually easier to delete the plate and start again using the original polyline.

It would be nice if there was an option for the plate to retain a logical link to the original poly shape.

I guess it is silimar to how microstation parametric solids work.