Adding a 7th string to a six string guitar is probably an idea that has occurred and been tried repeatedly in the history of the guitar. You can search for history about 7-string guitars and discover there had been a 7-string guitar in Russia tuned to an open G tuning in the 19th century. It is not the only country or time period where 7-string guitars can be found. But the person responsible for popularizing the 7-string guitar in the modern age was .
George Van Eps came from a musical family who also had a background in clock making. He created the detailed drawings of how he wanted his . Epi Statopoulo of Epiphone thought it a good idea and made him a guitar. But George’s real influence was on other guitarists came from his style of playing. This encouraged many jazz guitarists to go to 7 string guitars. Among those who play seven string jazz guitar are Bucky Pizzarelli, his son John Pizzarelli and George Van Eps protege, Howard Alden.
Geroge Van Eps and Howard Alden playing Night and Day on 7-String Guitars
There are a number of different tunings use for 7-string guitars. George Van Eps seems to have added a low A to give AEDGBe while many add a low B to give BEDGBe which keeps the interval between the low B and E as a 4th, the same as between the high B and e. Howard Alden added a high A instead of adding a bass note.
Epiphone never had a seven string in standard production. Their 7-strings were custom orders. However, Gretsch did make a George Van Eps model 7-string for a while. I do not know of any guitar maker who has 7-string, hollow-body, archtop jazz guitars in production. However, many luthiers will make custom 7-string models. I have seen moderately priced Eastman 7-strings guitar listed online.
Lenny Breau had a seven string solid body guitar built for him with a high A string. Eventually, Steve Vai tried a seven string but discovered the high A string broke to easily and switched to a low B for the seventh string. Solid body 7-string guitars are in production from Ibanez and have been used by metal bands.
Lenny Breau on solid body 7-String
Steve Vai 20th Anniversary Model 7-String