Hi Tim
Have you used the break line as part of a Surface element?. This can do what you are describing I think. You basically place some points along a path and if you leave the elevations blank they get interpolated from other points in the break line based on distance. Basically a path set by 'heights' is a break-line in the center and an extrapolated outline. So you could achieve this with surfaces that are made up of a break-line and an outline, until we upgrade the path this might be a good workaround for this challenge.
V3.20 is coming out this week which has some usability enhancements to the break-line so it might be worth having a play around with break-lines to see if it does what you need.
We are planning to enhance the way you create paths as at the moment you create them and retrospectively add the levels\depths. We think the break-line creation system would be superior and so we could implement this additional functionality you need.
Thanks for the great feedback as usual !
-Ted