Panasonic used to make affordable motorized pots. I'm not seeing the low-price ones at DigiKey today. They do have a $50 motorized slide-pot. You can also salvage motorized volume knobs from 1980s stereos.
If sound is too loud, apply a negative voltage to the motor to turn the knob and volume "down".
When sound is not excessive, but was, apply a positive voltage to the motor to restore the original gain.
If motor speed is proportional to voltage, then attack/release slew rates may be set with voltage stops.
An advantage: this limiter pot may also be your main gain pot, keeping the audio system simple.
Speed: IIRC the slew was OTOO 20dB per second. A smooth manual fade. This might be OK for a slow leveler. It is far too slow to limit transients. The motor voltage could be raised above specs and speed would increase. Motor would not overheat because the over-volt period is short. But 10X voltage is still only 200dB/Sec or 0.2dB/milliSecond or 2dB in 10mS. And 10X voltage might cause arcing in the brushes and rapid pitting and early failure.
There are fastor motors, and Mechano or robotics hardware to couple motor to pot. But electric motors rarely hit the millisecond speed we need for rapid audio limiting.
We could try voice-coils. Lower mass. However a long-travel voice coil's self-mass gives about a millsecond max response, slower with pot mass, and probably leverage to convert to wide rotary swing (or long linear swing) from short linear motion.
Air pistons and valves are a bit quicker.
All fast mechanical contraptions are prone to be clunky so may have to be isolated from the monitor room.