In LDT I would create 3D Polys (by Elevation) and select ".G" at the prompt
for points to select the AEC point object. That way the 3D poly will
automatically grasp the elevation, then I would type ".G" again to exit the
prompt. Is there a way to do this with feature lines in 3d? When I draw a
feature line and type .G or .P, nothing happens. Or do the points need to
be at actual elvations?
In Civil 3D you should use the Transparent Commands; for example '_PO for points. It is more convenient to turn on the Transparent Commands Toolbar and use the transparent commands from those tools.