As an example, I can think of the following:
With two quantization levels, I assume that the signal DC level is 0, and that the binary quantized numbers are
00 = signal positive, level OK
01 = signal positive, level too high
11 = signal negative, level OK
10 = signal negative, level too high
So AGC would be:
a. Appearance if either 01 or 10 means reduce the gain
b. If there is no toggling between 00 and 11, increase the gain
Now you can design a digital machine that detects these conditions, and feeds a DAC (two bits should suffice). Feed the DAC through a narrow enough LPF, and your loop should be fine.