I am curious to know if a multiplexer could be used along with a set of external band pass filters could be used to create the same effect as the commonly available IC's on the market. By having each filter go high if the tones are "heard" and then stepping through them in a circular pattern with a simple looping binary counter could we create enough points that can be used to differentiate very subtle differences in the audio world around the "ear" ?

For those not familiar with band pass filters I recommend reading https://en.wikipedia.org/wiki/Band-pass_filter

For those not familiar with a Multiplexer or MUX, I recommend reading https://en.wikipedia.org/wiki/Multiplexer

The counter could be powered by either a micro-controller or you could create one using several flip flops and a clock pulse.

If you look at the above links, you can see the simplicity of this sort of system. No idea if it will work, but I am making a note here to remind myself to give it a try :)