can't you just put a spot light behind it?

or are you going to be working on the image in post? If so I would do it as a separate pass:

Create an outline of your object from face on e.g. looking directly at it on the wall. Then hide the original and render the matte of that object on th wall. Then you need to render the sign alone without the wall. And then the wall without the sign (don't hide it just turn primary rays off). Then you can blur and fade the rendered matte of the sign put it on the wall, then comp the sign over the top. this way you can control the colour the sign glows without re-rendering, it will be a quick render to get the matte and you have a lot more control over your final image.

This saves you having to worry about colour bleed etc in MR, you can do it cheaply and easily with my way. Post again if you don't understand what I mean.

A