How can I use an image texture as a guide for weight painting? So I have an image of a map which I would like to use to paint vertex groups (for terrain feature particle systems) on certain areas of an object (a sphere).

I have mapped the map onto the sphere.  But even if I am in material or textured view, when I go into weight paint mode the object turns blue so I can not see the map so I don't know where to paint.

My next idea was to have a second, slightly larger sphere (in the exact same place) with a semi-transparent material of the map on it such that I could see the map on the large sphere and the weights underneath on the sphere I am weight-paining.  But it seems that material view does not show cycles transparent shaders. so the transparency on the large sphere shows up as opaque white and I can not see the weights on the sphere underneath that I am painting.

So is there a way to do this?  Either by showing textures/materials in weight painting mode, or by showing the transparency of the material on the large sphere?  Or some other way I haven't thought of yet to use an image as a reference to weight-paint on an object?

By the way, this is not a duplicate of this or this, both of those answers only work for BI materials it seems, I am using cycles.
Here is my solution:
Create a plane and subdivide it (as many times as you like, depending on your weight paint resolution requirements). Unwrap the mesh. Now you want to create a sphere from a plane using shapekeys. See the Cegaton's answer on how to do it here: What is the best way to unwrap a sphere?


When your transformation is ready, check the Basis key and add your heightmap image as a background image. Scale the added image to fit the size of a plane and check the image display type to Front. Set the Opacity value as you like.


Now you can easily paint in Weight Paint Mode, seeing a heightmap texture.


Transform a plane to a sphere using shapekeys.


As you can see all work quite nice.