Any tips on removing permanent marker from a guitar
You could try rubbing alcohol to see if it will remove the marker. If a laundry marker was used, the problem will be IF the ink penetrated or yellowed the clearcoat upon removing the signatures. A paint marker can typically be rubbed off with elbow grease.
Compounding polish might do the trick as well, depending on what was used. If it's permanent marker, you might have to *get it buffed out. If that fails (or you want a DIY project) you can wet-sand it down gently with 600-800 grade paper, then buff/polish the clearcoat. One problem is keeping the finish uniform... you don't want a rough patch (where you sanded off the sigs) or a polished clear spot (where you removed the sigs and then polished). If you polish, do the entire guitar back (and possibly the front) while you're at it.
Hopefully others will provide alternate ideas... glen