I'm intrigued as to how this works. I wouldn't mind a similar unit, but a standalone plug-and-socket design, that you could plug in between the Datasette and the C64, so that you could use it as a service tool, to adjust the tape azimuth and optimise the signal strength on Datasettes.
I reckon that Tabuhuso's unit is probably taking the tape signal after it's been through the the op-amps, then feeds it into the piezo speaker and the meter in a series circuit, using a variable resistor to balance the deflection of the meter. The output from the meter goes back to the Datasette connector to complete the circuit. If anyone else could comment on how this might work, I'm interested.
I have a scrap C2N I can hack apart for the male and female tape port connectors, so I'll look to pick up some parts for a play around on some veroboard.