The documentation I saw suggested that if the voltage reference was removed you should install a 47k resistor (R16) pull-up onto the cathode pin. The board I have, has values but not component designators. I suppose if this is the case an 0805 resistor could probably fit between the terminals, since I think one terminal of the TL431 is hooked to VCC.
Interestingly, from what I can tell the MCP1702 guarantees only 0.4% accuracy, whereas the LM4040A should be accurate to 0.1%, while the TL431 is accurate to 0.5-2% depending on which one is used. I'm a little surprised why the LM4040A isn't recommended. Unfortunately, the pinout appears to be different, and it seems to require a pull-up of ~39K.