The model release is not valid and will not be accepted unless is signed by a witness. You should ask someone around the place of shooting to help you, or maybe travel with a companion? :)
Or you can try to shoot silhouettes with no details in the dark areas, most of them would not need a model release, if the pictures are generic enough and the model can't be recognized by face or clothes. However, having them accepted is not guaranteed, it really depends on the pictures.