One would use the edge and some point in space (not on line of edge) to define the plane. Next click is the direction of offset. Then the value input. Sdmitch already made such a plugin, if I remember well.
@barry_milliken_droid, i agree that the inference sequence isn't that simple to a new user. Once one sees how SketchUp uses inferencing (we all need to learn that anyway) and memorises the last focused entity, then it's not that difficult anymore.
I just wanted to point out that an edge bounding a face has its "simple" offset with the 'Move' tool. Endpoint move at 90 degrees in plane.
As for offsetting arcs, the tool should have two options:
- to result in a curve (as it is now) and
- to result in an arc, maintaining the arc properties. That would be "fair"
Imagine the complaints when one needs to cut off both end segments of the offsetted arc instead of getting the curve with amputated end segments straight away. So there should be both options to choose from.