As always, it depends on your engine. Writing such a shader is not a problem, the issue is how to integrate it into your engine. Here is what I have done in my engine to highlight objects (deferred engine):

1. ensure, that your engine is capable of rendering emissive pixels

2. extend the standard model pixelshader (which renders the color of your models to the color/material buffer):

- add a color parameter to the shader, so that you can choose an optional emissive color for each rendered model

- calculate the emissive color intensity by taking the normal into account (eg fresnel or just normal-z value)

- add color.rgb * color.a* normal_dependent_intensity to the emissive buffer

If you dont want to hightlight an object, just set the color to black or the alpha value to 0.