I do not think it is the actual knock sensor that is (always) engine specific. Its more like there is a specific list of engines it can be used on, like Robert is pointing out. Example:
Look up the knock sensor for a 95 C1500 truck, 5.7, auto trans.
Now look up the same for a 95 C3500 7.4, auto trans.
Now look up the same for a 91 camaro 5.7.

Notice anything? They all list the same part number knock sensors.
ACdelco 21392
Airtex/wells 5S2167
Delphi AS10016

So if a 95 5.7 truck, and a 91 5.7 camaro use the same knock sensor, and both engines are the same displacement, what is different about the knock filters? Could they interchange successfully? I do not seem to have too very much trouble finding V8 TBI 7427's. I'm not just falling over them, but they are far from hard to find. Unlike the V8 TPI memcals that are made of unobtainum. So could you simply jumper the TBI memcal for MPFI, replace the chip with a EEPROM and have a useable V8 TPI memcal?