Looks very very dated, but could be useful. Screenshots would be good for those. Thank you for posting.

pikachiu
archive from <http://pikachiu.org/>
A Perl CGI script that lets you write in the speech bubble of a SJIS Pikachu. This script dates back to 2002.

moyashi
bean script from <http://ntak.net/>
Some perl site. Looks pretty big. Don't see any php here.

yakuba_kai
archive from <https://t-jun.kemoren.com/>
3 php scripts.

moepic
archive from <http://siokara.que.jp/>
A huge single file php script.

siokara104
archive from <http://siokara.que.jp/>
Huge php script, I guess for imageboard and textboard.

img
archive from <http://siokara.que.jp/>
Again a huge php file.

hokan_kai
archive from <https://t-jun.kemoren.com/>
3 php files.

https://github.com/hackpaint/pikachiu/blob/main/cgi-bin/jcodeLE.pl

>elsif (/(^|[\000-\177])[\241-\337]([\241-\337][\241-\337])*($|[\000-\177])/go) {
># 'sjis' jis 半角カナの奇数個の連続

Hm.

I heard github hates files in shift-jis codepage, did you convert everything?