Perhaps even more embarrassing, in recent times I was unaware that Sargent created some studies for it, including a watercolor now held at the Isabella Stewart Gardner Museum in Boston. I've visited the Gardner, but (once again!!) failed to notice it (that is, if it had actually been on display at the time).
Here it is:
A rambling discourse on Sergent's painting and the substance ambergris is here, and some supporting images are here.