My teacher Gene Clark was building true 8 string models in the mid 1960's. He was not the only one, the concept was in the air for players who wanted an instrument to transcribe mostly baroque music, so the two lower strings could be utilized a tuned in a variety of ways. Lot's of 8 string guitars were made on modern large pattern plantillas ( so define modern ) that could be used in reentrant tuning or diatonic variants. The Brahms came about because the reach of tuning between lowest and highest string needed a change in scale length. Before that true modern 8 strings existed, but the tuning was almost always standard tuning with two string added- The tuning did not call for a change in scale length, and variation in tuning could be as easy as descending chromatic, or a low D plus a reentrant string over it...it all depended on the players intent to transcribe to a certain key.
There was also nothing stopping anyone from making a guitar in G tuning or A tuning, except buying the right gauge strings. An 8 string guitar with the high E exchanged for a G string stepped higher is very possible. A 'G' tuning in guitar intervals put the guitar in 8 course G lute range. Lower the third string by a half step and you have a lute in G, but as a modern guitar. So the Brahms concept is extended by reaching the low bass. But wait, we already had that, it's called a Theorbo. Which can go even lower than a Brahms guitar. But why? Now we have pianos. You want to play Brahms? Just get a theorboed guitar, bigger range. Nothing is really 'modern'.
The Brahms guitar is an amalgam of new and old ideas, mostly old ideas- The end pin was an invention that the famous cellist Duport became famous for. He was quite fat and the end pin helped him hold the cello, for one thing. The end pin slowly became a standard for the cello and by about 1850 it was solid. So the end pin is a old idea. The multi scale concept is 300 plus years older than the invention of the end pin. So what's modern mean?
The Brahms guitar has range, but to me it's more about the change in technique and how the player engages the release of the string. For that matter the viola da Gamba was played picado style and with fingered arpeggio, so again there is a historical connection. Range is not the main issue, we as makers could already achieve the range by the era of Monteverde, around 1580- 1620. The thing that makes Brahms concept modern is Galbraith's personal reinvention of guitar technique, and that was made possible for him by using older design concepts in service of flipping the guitar up into a more vertical playing position.
Who invented it? Who knows. It was probably a case of simultaneous development. For that matter the Manuel Ramirez shop made extended range guitars in the early 20th century. They were popular with some players.
Pythagoras invented it.
Patience at the bending iron pays in rounded dividends!