It pains me to use this as a reference, but the Star Wars Holiday Special contains a scene where, after crash landing in a Y-Wing on a planet, C3PO and Luke are attacked by a monster. Luke ejects the cockpit from the ship. The ejection launches the entire cabin from the rest of the chassis.
Other ships used by the Rebellion/New Republic were also able to eject their cockpits: the B-wing and K-wing were capable of separating their cockpit for pilot safety.
A similar ejection mechanism may exist for the X-wing, where the entire cockpit is ejected. I can't find canon to support this notion aside from Biggs' quote and the ejection harness, though. Below is a cross section of an X-wing, which does not show any ejection mechanism.