[firedrake] Mapping nodal values from parent to extruded mesh

Hi all, I'd need to copy nodal values between fields on parent and extruded meshes. For example, copy 2d->3d (constant over vertical) or 3d->2d (extract surface/bottom level). The horizontal function space is the same. Is there an easy way to do this? Is there a map of extruded nodes somewhere? Thanks, Tuomas