Marc, I don't have an incident meter, but I wouldn't take any readings off of the sun. I think you would do as you have said, but slightly differently. Point the meter at the camera for overall light falling on the subject. Find the lowest value which needs detail and read this as a shadow value. This can be done by using your other hand as a shade to find low values. From what I've heard, the shadow values are the tricky ones, because the reading is a bit more subjective than a reflective meter's reading of light bounced off of a darker area.
I'm sure others will correct me on this one, but I think this is basically correct. My understanding is that the lower the scene's contrast, the more experience and judgement play a role with incident meters. tim