Mark,
Thank you for contacting National Instruments! When you configure an RTD channel in the Measurement and Automation Explorer (MAX), you have the option of choosing from a list of "standard" RTD's or entering your own polynomial expression as the temperature coefficients. Therefore, negative coefficients shouldn't pose a problem. Also, I don't see any reason why a silicon diode temperature sensor would pose a problem either.
I hope this information is helpful to you!
Jeff P.
National Instruments Applications Engineer