Is it possible to affect the emissive shader with the color ramp at all? I would like to do something like this (image below):
You could use the supplied Gradient node, mapped appropriately..
Here, just using the Generated texture space on a plain mesh circle.. (it could be a sphere, if you want it to be) The Mapping node rotates the gradient to the right orientation, the Color Ramp converts the input 0-1 value to a range of colors.
(With a bit of Glare added in the compositor to simulate the reference)
That can be done using the geometry input node and doing that you'll have to tune the Z value depending on the object geometry:
The node math is done this way, as in this example Z is going from -1 to +1. So dividing by 2 obtains something between -0.5 and +0.5 and add 0.5 obtains something between 0 and 1 which is what we want for the colorramp.
You can also use a UV map (to avoid geometric calculation). For instance make the UV map with 'project from view (bounds)', so that you can: