I am trying to model a car seat. I would like to be able to Grab the selected edge (marked with the red arrow) and translate it to an arbitrary location. However, I would like to constrain the median point of the edge to the plane that goes through the middle of the volume below it, on the Z axis, parallel to its (large) faces, i.e. not move the edge "outside" the volume.
Unfortunately I cannot use regular constraining, because the volume in question is rotated from both the Global and the object Local axes. Also, the normal space of the edge is not aligned to the normal space of the volume.
I am thinking I can do what I want either by (a) constraining the movement to the YZ plane, in the normal coordinates of one of the large faces of the volume below, or (b) by snapping the vertices of the edge being moved to the projection along the Z axis of the mesh geometry.
How can I do this? Is there any other (blender-preferred) way to achieve this?