I tested my simple aiptek tablet with a smartphone. It worked better than I expected. So good in fact that I am convinced a standalone bluetooth pen. Capable of drawing on a table. Acting like a drawing mouse on the smartphone screen. Is a very good product.
If you make it as cheap as a pc mouse than I believe it could be a very important accessory to the phone. Drawing on the phone screen is not very smart since the size is so small. Better to use the whole table area and a blue tooth ?mouse pen. Or like a drawing tablet cover like the below image.