Motor specifications - how to compute allowable running time for power levels between the continuous and peak specs?
For a brushless motor the length of time it can be run at peak torque/current is simply a thermal limitation issue, even down to very low speeds. Provided you don't exceed the safe operating temperature of the motor, there is no fundamental reason why it cannot be run at peak torque continuously.

If the motor has a temperature sensor then that would seem like a good solution to your problem. However, some things to keep in mind are exactly where this sensor is located, and what the thermal resistance is between the motor coils and the sensor. This could be considerable. You can do some test to determine it by measuring the coil resistance (which is a essentially a direct measurement of their temperature) after running it at your desired torque for a while, and comparing this with the temperature given by the sensor.

Keep in mind that motor manufacturers are usually quite conservative with the peak torque operating conditions, as they cannot know the physical environment the motor will be installed in. If this is a closed cabinet, with little air circulation and a plastic (thermally insulated) mounting, then it will be substantially more arduous for the motor compared with it being bolted to a large aluminium mount with good airflow around it. Also note that the power loss rises with the square of current/torque, so there will be substantially more heat (~3x) that needs to be removed at 1.75x torque levels. This is yet another reason why manufacturers will be very conservative.