By George I think I've got it.
As noted in a previous thread, those of us that buy a used Vanguard raft from Greg at Timberline Tours have a duty to remove the Timberline lettering on the large yellow plates on the bow and stern. Other than flexible urethane paint, which tends to pricey, it has come to look like the only way of obscuring the lettering is to glue a PVC fabric patch over the 'Timberline'.
I was in the process of doing that today and was cleaning the yellow plate with Denatured Alcohol (DA) when I noticed the black paint started to come up. Turns out if you saturate a rag with the DA, let it sit and soak on the letters, and then scrub vigorously, you can remove the black lettering. And, mirable dictu, Denatured Alcohol is not a solvent that attacks PVC fabric, in fact is frequently used as a cleaner with it.
This does raise an interesting further question. The paint used was flexible urethane, very closely related to the stuff that is used in boat treatments. In addition, DA is frequently used as a camping stove fuel.
I wonder what happens to the expensive boat treatment if you spill camping stove fuel on it...