Is there a way to monitor the validity of converted data in Clock Mode 11 for the MAX1230 ADC?

No, the MAX1230 ADC produces an always high EOC signal in clock mode 11 and therefore doesn't have a dedicated signal that can be monitored to alert the user of the availability of a valid conversion result. However the DOUT signal will match the /CS signal, going HIGH when the /CS signal goes HIGH and going LOW when the /CS goes LOW thereby offering a method to ensure the DOUT line isn't stuck in a faulty output state.