Not a rag expert here, but I love a challenge.

My recollection is that you're right: Scott Joplin wrote in minor not so much. But if you stray from the North American continent for composers, you may have somewhat better luck.

I've been having fun YouTubing all morning on this theme; here are my results:

A familiar Spanish classical guitar piece (whose name I can't bring to mind) done ragtime style is at
this page.

And here is a medley of guitar rags, several in minor.

John Renbourn wrote a minorish rag piece called "Faro's Rag."

I loved this piece by Lasse Johansson, called "Russian Rag."

Finally, I imagine there are lots of klezmer compositions from the ragtime era that could be adapted as guitar rags, but I couldn't name any particular ones.

I hope that helps! Thanks for giving me a fun hour or so.

Claire