I doubt you would have much of a legal issue for one guitar, maybe an ethical one. Even if Schecter did take legal action, the most they could do is issue a cease and desist letter telling you to never do it again. They could not force the owner to have it changed.
If you want to play it safe, you could put your logo on as long as it said, in small type, "Customized by". You could go further and use something like "Schecter customized by". No one except total guitar geeks is going to notice the small type. With them you would be displaying your honesty as well as your work.