Digital to Analog Temperature Sort/Convert

Hello,
I am currently working on a heating system that cures out thermoset plastics with a Watlow F4 ramping controller.

Inside the vessel there are 16 items that are temperature monitored and datalogged through an independent datalogging system operating through RS232 with J-type thermocouples.

My process would benefit from driving the Watlow controller by the hottest temperature, however, I can't predict which thermocouple it will be due to the nature of the variable sizess of items being processed.

To resolve this issue, I am considering splitting off the datalogging line to an RS232 shield and using an arduino to sort and output the highest temperature to the Watlow as an analog signal.

Sorting the values should be easy enough, but I am wondering how feasible it would be to convert that digital signal back to it's unprocessed analog state for the Watlow to interpret.

Are there any other issues that I may be running into?

Thanks

Additional notes:
Watlow also supports 0-5V or 4-20mA sensor inputs as well. This signal is probably a more suitable output for arduino to process instead of mV.

This still leaves me with determining the proper digital to analog conversion to ensure the arduino output is reflective of the raw input from the hottest item.