Recently LMB-Drag was made to move the cursor (edit, this has since been reverted).
However this just moves the cursor under the mouse,
it _could_ be useful to use transform here, it allows.
- Drag, XKey to lock to an axis
- ... Shift+Y to constrain to a plane.
- ... Hold Ctrl to snap (snap to verts for example is handy)
- ... Shift for more precise placement...
... most other transform keys.
One reservation I have about this, is that you probably would often want to be able to snap to vertices and edges, and not have to depend on the current snap options in the 3d view header.
So we could override the snap options that transform uses.... but this seems a bit crappy too, since you may want to snap to faces in some cases, or even snap to grid.
As an alternative, we could just not bother attempting to integrate this with transform, and add snapping to the current modal operator (similar to how we have with the Ruler operator, which shares low level code with transform snapping)
But in that case we miss the axis constraints which seem like they would be nice to support.
Here is a test patch to show how it works: P87
Note, it crashes if you try to scale and its hard coded to LMB for cursor place, so its mostly a proof of concept.