Don't mock the LED-LDR invention
. It's fantastic, I remember swindleing top marks for a lab project at Uni using one of those. It is cheap, easy to make, easy to use, but not amasingly linear, but that is easily worked around. If your after a quick and easy bodge, the LED/LDR solution is for you.
Alternatevely, why not just use a digital pot. As linear as you like. No distortion and pretty much analogue, I'm pretty sure not many people can tell when a 0.07dB increace in gain happens. I've made a super high quality AGC using an op-amp and a digital pot before, (and a bit of control circuitry to get the 'A' bit of AGC).
You can buy voltage controlled resistors (old audio ones are basically big wippers with a small motor), you can also get more modern opto-isolators/couplers that give the same result as the LED/LDR (but better), but they are large and/or expensive.
It is quite easy to interface a simple steady voltage signal to the inputs of a digital pot, there must be hundreds of ways. But remember, if your after a quick and easy bodge, the LED/LDR solution is for you (sand off the domed part of the LED, and then superglue it to the LDR once you've sanded that as well to get a good key, plaster it in tipex or insulation tape, and there you are, A voltage controlled resistor (not really but as close as you'll get it's actually a current controlled resistor
)).