In the first instance I would be tempted to try isopropyl alcohol wipes (Sterets or any other similar brand of medical swab should do fine). In a brief experiment after reading this thread, I used a black Sharpie marker to make a small mark on my acoustic. After half an hour to dry, long after it was smudge-proof to the touch, a Steret took it straight off. Then a quick rub with a dry cloth to take the alcohol itself off, and there is no sign that a pen ever touched the guitar.
I also dug out an old skateboard deck with some Sharpied writing on it that I had in the cupboard, which had been there several months - the alcowipe again removed the black ink without any visible damage to the underlying surface. The only issue would be whether the ink has reacted with the finish during the time it's been on the guitar already, which may be possible with a nitro finish if that's what it has.
