The only intentional underslung jettison I've actually seen was a Holden Kingswood sedan dropped from an RAN Westland Seaking at a NAS Nowra airshow in 1998. There was a great video that the AMTDU instructors showed us on basic course of an Army Unimog truck dropped from a Chook at a DZ in western Sydney. It had been incorrectly rigged so that the two rear strops rubbed together and snapped, allowing the truck to swing forward. The crew then sent it off on it's 200' free fall with predictable results.
There were also rumors about an incident relating to underslung boat trial that was being conducted by AMTDU in the early noughties. Apparently the vessel (a large RHIB) impacted the underside of the Chinook during flight trials and was cut away! I was busy with other stuff at the time so I don't actually know the story.