Would definitely be a difficult case from the standpoint of:
1.) It is a motorcycle and I am fairly certain the stock seatbelt is not DOT approved leaving that questionable.
2.) If you add a non DOT approved seat belt for racing or off road you assume the liability unless you could prove a manufacturing defect and that is negated if you drive on a public highway.
3.). Vehicle is not crash tested to prove whether the restraint would prevent or aggravate an injury.
DEFINITELY a tricky case to put in front of a jury!!