Even when using the ImageIDRequest trick shown in the documentation, the images are still not transparent:
procedure AddImageListToImageEnMView(ImageList: TImageList; ImageEnMView: TImageEnMView);
var
  i: Integer;
begin
  ImageEnMView.Clear;
  // Add an ID for every bitmap in a TImageList
  for i := 0 to ImageList.Count - 1 do
  begin
    ImageEnMView.InsertImage(I);
    ImageEnMView.ImageID[I] := I;
  end;
end;
procedure TForm1.ImageEnMView1ImageIDRequest(Sender: TObject; Index, ID: Integer; var Bitmap: TBitmap);
begin
  // Retrieve the image from a TImageList
  Bitmap := TBitmap.create;
  ImageList1.GetBitmap(ID, Bitmap);
end;