Well, yesterday I gave them a call. Very friendly people. Turns out that back in the early 1970's, the artists were using a water-based ink that probably isn't even legal anymore. Anyway, the discoloring that I see is probably compounds from the ink leaching into the linen fabric. The only option to prevent this would be to set the ink in a large, hot oven which isn't an option because the print is too large for any oven I have access to. I do have access to a few kilns, but the only large one is a wood-burning one and somehow I don't think that would work. Hah!
Anyway, I did try out the bread technique and by golly, it worked pretty well on the unpainted surface. On the inked area, however, the bread wanted to lift the ink a bit.
And there you have it.