My idea is similar to the Aussue disco. I'd put the cut and the locking sleeve on one end of the bar, and I'd lock it with a spring loaded pin that engages both halves of the locking collar. The spring is loaded in the lock direction by the lever action, and the pin will engage when the holes aligh the first time after the lever is engaged. Disco works the same way, loading the spring in the undo direction, and it comes loose the first time the pin unbinds. The locking sleeve is two concentric cylinders, one each locked to the bar on each side of the cut. I'm still leaning toward welding the sleeves, but I'm a little leery of loss of metal temper and strength at the welds. The pin will only be into half pin diameter in the bar to maintain the most bar possible.

The Aussie disco had a manual pull pin which required you to crawl under, or an optional air cylinder actuator for remote operation, but that requires an air sourse.