Site-wide shift jis support could be implemented using a new BBCode. In CSS:

@font-face {
  font-family: "saitamaar";
  src: url("Saitamaar.woff2") format("woff2");
  font-display: block;
}

.shift_jis {
    font-family: 'saitamaar', 'MS Pgothic', 'MS Pゴシック', IPAMonaPGothic, 'IPA モナー Pゴシック', Monapo, Mona;
    line-height: 15px;
    font-size: 14px;
}

Saitamaar is a great font for that sort of thing as it preserves detail MS Pgothic doesn't on modern Windows. It's what https://aahub.org uses.
https://aahub.org/assets/fonts/Saitamaar.woff2