Make it possible to edit on a plane such that clicking in an empty space adds a point on that plane in that position and points on that plane can be dragged within that plane. Also, clicking a point not in the plane shifts the plane to the parallel plane intersecting with that point.
Also a button will be used for toggling editing within the normal 3d space or a specific plane.
There are a number of options for how to represent an editing plane graphically. The most attractive options would be minimal, yet provide sufficiently clear visual information.
I suggest creating a crosshaired outlined box in the plane around the center axis. Shifting the plane to another point will actually center the image on that point.
<hr /><i>dir7 added on 2013-01-16 19:36:35 UTC:</i><br />
I feel this would add too much complexity to the interface and contradict the common mode of editing already established.
Instead we hope to add the ability to generate geometries from bitmap drawings.
However, using the mouse can really be a preferable way to editing drawings. There may be some more thought put into this feature in the future. if it can be integrated in a very non-disruptive way.