Most of the digital cameras we mortals can afford have a "closest focus" at about a meter or so, give-or-take a few centimeters. Unless you've got something pretty special, all you're likely to get by sticking the camera inside is a big blur.

Digital cameras generally can use flash to take the picture, although the built-in flash on most lower priced ones is pretty puny. Quite a few of them have an optical "autofocus," and need enough ambient light to "see" the object in order to focus on it; and in low (or no) light situations they tend to not get in focus and produce mostly fuzzies - especially at "close-up" range.

If you can get a camera where it can "see" the inside of the guitar, you can probably get an eyeball pretty close to the same place.

John