A transfigured port key in his glasses does seem possible, or some kind of explosive device hidden in the transfigured ring.
EDIT: Fawkes is also a way out here, if Harry can delay an enemy attack for several seconds
Second edit (the serious one): The ring contains some kind of binding device. This could be the blinding potion Harry used in battles, a set of flash-bang grenades, or (most likely) a seventh-year variant of the blinding potion Harry bought off a student. This countermeasure will blind the unwarded death eaters which allows Harry to immediately fall to the ground and call Fawkes. Fawkes teleports Harry out and allows escape.
A transfigured port key in his glasses does seem possible, or some kind of explosive device hidden in the transfigured ring.
EDIT: Fawkes is also a way out here, if Harry can delay an enemy attack for several seconds
Second edit (the serious one): The ring contains some kind of binding device. This could be the blinding potion Harry used in battles, a set of flash-bang grenades, or (most likely) a seventh-year variant of the blinding potion Harry bought off a student. This countermeasure will blind the unwarded death eaters which allows Harry to immediately fall to the ground and call Fawkes. Fawkes teleports Harry out and allows escape.