Yep, NinjaPad in a ZIP. Several stumbling blocks: you have to set "kind of project" to HTML, or Itch won't show the "this file will be played in the browser" option for ZIP files. The index.html file genuinely has to be in the root of the ZIP file, not in a folder inside the zip file. And just to make things interesting, NinjaPad may not run locally, because of browser security rules... so my test environment was uploading stuff and hoping that it worked.
Should I be worried how often the least convenient part of retro development is modern websites? GBDK is powerful, cc65 is fast, Open Watcom is flexible, and it once took me three days to send an executable over GMail. I had to rename it attack.com... because that's not suspicious.