Yes, the __toString is really just a magic method and a simple casting should work just as well. In your example it could be something like: (string) $value. String concatenation also works. Have you considered something like the following?
public function i_set_the_field_to($name, PyStringNode $content) {
return array(new Given('I set the field "'. $name. '" to "'. $content. '"'));
}