question

To extract the equipment from your question:

1. BMV-702

2. Cerbo GX

3. Victron temp sensor

4. MPPT (Victron?)

Indeed you can messure the midpoint voltage of the batteries with the external input of the BMV-702 shunt. In this case you can set an alarm when the midpoint voltage deviates too much. But in this case you've used the external input of the shunt for the midpoint voltage, and therefore can not be used for a temperature sensor anymore.

When you connect the Victron temperature sensor on to the Cerbo GX, indeed you can create a role in the Cerbo GX menu/software. In this case you could assign one of the Cerbo's relais connections, and make a minum and maximum temperature in between which the relais should be closed or open. The Cerbo GX relais connections are limited to 4A maximum of current, so a bigger relais connected on to the Cerbo relais could be mandatory, to switch on/off bigger load/charge currents.

When you have a Victron Smart MPPT, you could also take a cheap Victron Smart Battery Sense module which is capable of both monitoring the battery voltage, as well as the battery temperature. If you setup a Bluetooth Victron "Ve Smart Networking" network, and connect the Victron MPPT with the Smart Battery Sense in this network, they can work together, and you can set the MPPT so that it will shut off at a to high/low battery temperature.

When using more than one battery, more than one Victron temperature sensor, or Smart Battery Sense could/must be used.

I'm not to experienced with the Cerbo GX, but maybe it's also possible to control a Victron Smart MPPT via an external temperature sensor on the Cerbo GX, directly in a Ve Smart Network, without using one of the Cerbo GX relais connections. Maybe other forum members will know a solution in this option.