Steve Stich states at today’s Crew-9 news conference that Dragon has a new contingency capability if all 4 parachutes fail; the SuperDracos will ignite prior to splashdown.
The Crew-8 return to Earth will also have this capability.
(He said this about 20 minutes after the start of the stream.)
Animation of a ‘land landing’: https://youtu.be/Cf_-g3UWQ04?t=1m32s