I have tried creating a simple content processor that assigns the custom effect I created to models instead of the default BasicEffect.

[ContentProcessor(DisplayName = "Shadow Mapping Model")]
public class ShadowMappingModelProcessor : ModelProcessor
{
    protected override MaterialContent ConvertMaterial(MaterialContent material, ContentProcessorContext context)
    {
        EffectMaterialContent shadowMappingMaterial = new EffectMaterialContent();
        shadowMappingMaterial.Effect = new ExternalReference<EffectContent>("Effects/MultipassShadowMapping.fx");
        return context.Convert<MaterialContent, MaterialContent>(shadowMappingMaterial, typeof(MaterialProcessor).Name);
    }
}

This works, but when I go to draw a model in a game, the effect has no material properties assigned. How would I go about assigning, say, my DiffuseColor or SpecularColor shader parameter to white or (better) can I assign it to some value specified by the artist in the model? (I think this may have something to do with the OpaqueDataDictionary but I am confused on how to use it--the content pipeline has always been a black box to me.)