Use a negative margin on the full bleed item? Bit hacky, but works fine:

  .content {
    max-width: 85ch;
    width: 100%;
    margin: 0 auto;
  }
  .content img,
  .content .full-bleed {
    width: 100vw;
    height: auto;
    padding-left: 50%;
    margin-left: -50vw;
  }