Probably there have been many private constructions of polyphonic guitars since the 60-ies. The first I know of is CCDB (Claudio Cesar Dias Batista), the sound tech of the Brazilian avant garde band Mutantes.

1984 was a boom of the separate string technology since guitar2MIDI converters became available from Shadow, Ibanez, IVL, Photon... but for most users they were not accurate enough.

Here is a recent article telling more about the story.

Here is the proposal Paradis made to Roland in 1991, its still amazingly valid