Even with a rocket deployed parachute you need to be at least about 200ft above the ground for it to work. So there's a dead zone during takeoff and landing where you're too high to survive a crash but too low to parachute down. Military VTOL jets like the Harrier and F-35B address that risk using zero/zero ejection seats which can launch the pilot straight up high enough for a parachute to work but that's obviously not feasible for general aviation.

Since ejection wouldn't be feasible, wonder if an undercarriage airbag, perhaps like bumper rails to keep weight down and work around landing gear, would be effective in a crash?

If the aircraft is falling out of control there's no guarantee it will impact the ground with the undercarriage side down. Plus airbags would have to be huge and heavy to do any good.