Has anyone ever heard of one. I tried a google search but came up with nothing. I think it would be a useful tuning aid. A gauge could give some kind of scale indicating degree of knock and a digital output could go to your ignition box and retard timing.
I've worked with several aftermarket knock controllers in an engine dyno environment on a variety of engines, and in my opinion, there is no foolproof knock sensor on the market yet.
The best knock detection systems I've seen use pressure transducer spark plugs (or transducer rings located between the plug and the head) and measure combustion pressure directly. These are very expensive systems, and therefore not very practical unless you're doing engine development for a big budget manufacturer. These systems will map the normal combustion pressures and track them versus crank position and cam position. Then, you can set upper and lower bounds for normal operation so you can flag errors if the pressure spikes or dips when it shouldn't. Fancy.
The simple block-noise transducer approach to knock sensing works poorly. If you gate the "listening" to a particular crank position and only listen for knock in likely areas where timing events occur it works better; that's how most OEM systems work. Even these systems are easily fooled, however. If you look in Fords lightning truck calibrations, the knock sensor stuff is actually turned off. Why? It didn't work...blower gear/rotor rattle sets off the knock detection. I know this because I've re-enabled it and done testing with it enabled.
Anyway...if I was trying to fit knock control to an engine and wanted to use aftermarket parts, there is only one manufacturer out there I'd use. http://www.jandssafeguard.com/ It's not perfect, but it's close. It works damn well, especially on hydraulic cam engines without roots blowers.