I think that I have finally found my answer:

// Create and cache a dummy material
if (_dummyMaterial == null) {
    _dummyMaterial = new Material(Shader.Find("Diffuse"));
    _dummyMaterial.hideFlags = HideFlags.HideAndDontSave;
}

// First ensure that all shader assets are loaded
UnityEditorInternal.InternalEditorUtility.SetupShaderMenu(_dummyMaterial);

// Fetch all shader assets
Shader[] shaders = (Shader[])UnityEngine.Resources.FindObjectsOfTypeAll(typeof(Shader));
// Filter out those that are supposed to be hidden
shaders = shaders.Where(s => s != null && s.name != "" && !s.name.StartsWith("__")).ToArray();

This appears to be a better solution than my previous one because it works without having previously selected the shader popup menu for a material. Not perfect, but as good as I could figure out.

C#

ShaderToImport = EditorGUILayout.ObjectField(ShaderToImport, typeof(Shader), true) as Shader;