As far as I'm aware, the only way to include them in your list would be to actually include copies of them in your project.

That's not such a difficult task though, simply download the Unity Shader Source Files and put them into a folder somewhere in your assets folder.

I've only briefly tried this to check that they actually do show up before posting this answer though, so there may be unintended side-effects - particularly if you make changes to any of the shaders. The reason being, because these imported shaders have the same name as the built-in shaders, they override the actual built-in shaders in your project (including the terrain shaders, particle shaders, etc) so any changes you make to these files in your project might have corresponding side-effects wherever the built-in shaders were used before!

Also, I think it may be the case that all the imported shader files will be included in your build, whether they are actually referenced or not (much like your script files), however they are relatively small so it wouldn't be much of filesize burden.

Anyway, hope this is enough of a solution for your needs :)

Someone figured it out - yay them!

http://answers.unity3d.com/questions/307421/select-shader-in-custom-editor-gui.html