Create and select custom transformations on the fly.

When transforming an object, you can use the transformation axes of another object or element, but these must first be added and selected in the Transform Orientations list. I suggest that while an object or element is being transformed, any element in the scene can be temporarily used as a transformation axis. In the same way that (B) is used to select the base point, (D) could set the direction. Enabling preselection in both cases, or displaying a hair on the cursor with the axis orientation before activating it, would help with the selection (tangent to edges, normal to vertex and faces).