I don't know if it is a new change which came with VI or not. Either way, I didn't get the opportunity to see this option (below) until recently.

If you get caught off guard and your ruler or a high-stars general ended up trapped under siege but your best relief force is nowhere near big enough to win the province back before he's starved out, you at least have the option, now, to activate a sally battle but click an option to abandon the province and escape intact!

For whatever reason, the AI never seems to pull this stunt when I'm sieging them....