Tom,
In answer to my own question,
a curved ramp made with the roof tool
will have it's lower inside corner lifted
out of level, so that's no good.
If you make a mesh in the form of a curved ramp,
copy it and paste it, and then de-elevate the pasted copy by the
thickness of the ramp you want, then SEO the original using the
second copy as the operator, you have your ramp.
If you then use the mesh to roof add-on to make roofs that
follow the slope of the mesh and use these roofs (keep them grouped)
to trim the top of the wall.
Delete the roofs and you will have the curved wall trimmed
and supporting the mesh ramp.
I've done this and it seems to work.
Peter Devlin