GD&L, jets with undermounted engines are designed so the lowest engines (inboards) are only a few feet off the ground. The reason undercarriage legs are as long as they are is to allow for rotation on takeoff/landing and to stop the lowest bits getting scraped- the engines. So to leave the engines undamaged by lowering the aeroplane, you have only got a metre or so at the most- not worth the technical expense of going to the trouble of a system like that.