It is not a maybe at all. Where you suggest sounds just fine. What you do is contact Princess well ahead at Customer Relations and place a request. If you booked via a TA, they should have the information and be able to submit it on your behalf. Just explain what you want to do and they notify the ship.
We just did this last October in Messina, Sicily, getting off before ship returned to Civitavecchia. We spent six days in Sicily, flew to FCO, and then out to Civi and got onto another PCL ship.
ETA: It is called a deviation request. AFAIK, you should let them know before you sail, but the post above indicates you can let them know on board. If so, I would get to Guest Services pronto after boarding. They do prepare a list of anyone disembarking early and pass it on to the authorities at that port. A Princess GS rep met us and we were clocked off the ship at the end of the day.
(Normally, such guests depart in the morning, but we left later in the afternoon officially).