Accessing I2C address of ADT7516 ( Temp sensor )
As you suspect, there is a chip in the way that might be preventing you to communicate with the sensor. The chip is the ADG774, I suspect it is U4 but I could not find the refdef in the schematic.

The IN pin of said chip must be tied low in order for S1A and S2A to be connected to D1 and D2. To achieve this, you have two ways: either you rework the board, forcing IN to gnd, or you find out how to generate such a signal with the big cypress USB chip. The PIN you must pull low is PD2.

Good luck!