QTR-Quadtone RIP

I have a nice job that involves reproducing a large series of 17th
century European Maps. I have done several nice variants of monochrome
using QTR that I like and am trying to sell the client on. However,
the original maps, although in excellent for steel engravings over 300
years old have yellowed quite a bit in the highlights. The engraved
lines are a reddish sepia.

In my many tests I have found no setting with QTR curves that allows
me to introduce this yellowish tone into the highlights and simulate
the paper yellowing of the originals. I have gotten very good
reproduction using RGB workflow but I would like to do these out of
qtr if possible. 

My understanding is that the QTR Ultrachome driver shuts off the
yellow for the purposes of greater longevity. But I am not 100% sure
of this.
Does anyone know?

John