[John Scheffel]

Which version of Modeling are you running? What kind of geometry are you trying to snap to?

I tried 3D Curve/Line and it seems to snap to 3D geometry (edges, vertices). For your example I think the challenge will be getting the centroid of the frame member because there is no geometry to snap to at the centroid of the end faces. It is easy to create lines along the center axis of faces that have an axis (such as cylidrical faces) using the By Axis option, but that wont work for cross sections with no defined axis. I don't know of any easy way to snap to the centroid of more complex cross sections. Hopefully someone else will have a suggestion.