after reading about the project on a German news site, you would like to let you know about MIMO (Musical Instrument Museums Online), which is a EU effort to provide a central access point to data about musical instruments in 11 European museums.
As it turns out, this seems to be an ongoing project still in its infancy, but it promises to be a great resource once ready, for instruments including but not limited to the guitar. In one or two years or so it should contain data about thousands of historical instruments, complete with standardized photos, sound samples and even videos.
They also plan to provide a critical database of instrument names across several languages.
Already the site can be accessed in English, Nederlands, Français, Deutsch, Italiano, and Svenska.
Check it out at http://mimo-project.eu/