The only real case I use switch fallthrough for is when you have two cases with the same code, e.g.:

  switch (foo) {
  case 1:
  case 2:
    /* common body */
    break;
  case 3:
    /* body 3 */
    break;
  }

It's not cognitively hard to make an empty case body fallthrough to the next run, but include an implicit break at the end of every nontrivial body.

Supporting Duff's Device is not a compelling feature to support--irreducible loops are basically going to destroy any hope of optimization you might accrue.