Tim,
It occurs to me that the voltage at DXx is not relevent to the 1.8v Vcc of the PEX-8732. The temp sensing p-n junction or diode in the PEX-8732 is not connected to any other circuitry in the device, it is simply a free-standing p-n junction. Temperature sensing is accomplished by monitoring the current through the remote device p-n junction, as shown here. So, DXx could approach VDD of the TMP device, however, it is only applied to the remote sense junction, not the internal circuitry of the PEX-8732. One useful exercise is to get the internal circuitry of the PEX-8732, verifying it is indeed isolated and not subjected to the 1.8v Vmax.
-Leonard