Can you register a used guitar that has never been registered before even if you are not the first owner? Possibly, but you shouldn't count on it.
You absolutely can.
"Warranty" and "register" are two different things.
Anyone who buys a Taylor, whether it's used
or new, should register it online. Why?
1 - It gets you signed up for Wood & Steel
2 - It associates that serial number with the owner, which makes the CS Reps job easier if the guitar ever comes home
3 - If the guitar is ever reported stolen, and someone tries to get service on it, it can be identified as stolen
4 - Makes it so you can get notified of Taylor events (Road Show, Find Your Fit) in your area
Basically, registering the guitar with Taylor doesn't mean you get warranty coverage. It simply identifies you as the owner. With very few exceptions, you'd still have to be the first owner of that guitar to get the warranty...