I finally found the source code to the Batman Returns project on Sega CD. Yay! I know I’ve been teasing you with the dead link on the archive page for so long, and I’m sorry. The package is now live and ready to download for you delight and amusement.
Please link to the page and not the file. For educational purposes only. Yadda, yadda, yadda.
Cheers and enjoy!