I think a motor/generator pair would be the most reliable solution. But if an asynchronous motor is used, the output frequency will be slightly less than 60 Hz, which should not be a problem if the 120V's are rectified and used as DC. But it might be a problem if the device uses the 60 Hz to do some type of timing/synchronization.
A motor/generator pair would also have the most graceful failure mode. If either the motor or the generator fails, the output voltage will gradually drop to zero. But with an inverter, you just can't know ... It could supply same phase 120V to all of the outputs, or some phases might disappear, leaving only one phase or two phases active. There could be other failure modes that cannot be predicted up front.