As soon as i will have time for this and overall picture how it should work. Actually, this feature is not even announced. Placement of sensor is not so good as it has no direct contact to FPGA chip. So, the measurement won't reflect actual temperature and will be affected by environment and even airflow direction where sensor will show 20C while real FPGA chip has 70C. Thus it cannot be taken in account when you judge the chip heating. You need to create a compatible environment to make measurement somehow reflect the actual chip temperature.
So, this whole complexity stops me from offering temperature measurement function. Users will relay on this while it may not reflect the actual temperature.
What about using a NTC and an I2C ADC ? The NTC could be put on the FPGA's heatsink and connected to the board with wire ? MCP3021A0T is cheap and 10 bits should give an adequate resolution for the temperature.
I know what is thermistor. Just didn't know the NTC abbreviation. It's not more convenient than AD7415. You can glue this chip directly to heatsink. I think it's possible to move the heatsink toward the RTC board with one rib precisely trimmed so it will touch the chip from the bottom and thus provide the heatsink temperature.