I just developed a similiar thing for my project and it works, but it's bad for debugging so I skipped it. What I did was implement this:
http://blogs.msdn.com/b/shawnhar/archive/2010/05/07/effect-compilation-and-content-pipeline-automation-in-xna-game-studio-4-0.aspx
I have an Engine DLL Library and in it I just created a resource file, called "Effects.resx" and added some simple effect files that aren't compiled then later with the code above implemented I just created a method for generating an effect.
public class DynamicContent
{
public static Effect CreateEffect(GraphicsDevice graphicsDevice, string effectCode)
{
EffectContent input = new EffectContent() { EffectCode = effectCode };
EffectProcessor processor = new EffectProcessor();
CompiledEffectContent content = processor.Process(input, new MyProcessorContext());
return new Effect(graphicsDevice, content.GetEffectCode());
}
}
then with my Resource file I just accessed it like this:
// "multitexture.fx" for example in the Resources.resx / Effects.resx
Effect effect = DynamicContent.CreateEffect(GraphicsDevice, Effects.multitexture);
Some things that aren't ideal about using the Pipeline is that it needs the full .NET Framework, so to be able to use the tutorial above you need to change the Target Framework by going into your Game Library Properties / Application.
This way, no files are loaded from the content directory but compiled during runtime and bundled with the Game Library as a Resource.