If you have your own music in just intonation format (https://ad8e.pages.dev/clipboardformat.txt), you can put it into the Record text box and the piano will play it. It'll put its fingering into the browser console. This fingering is pretty crude and doesn't consider overlapping pitches, but it works. You can see an example by Ctrl+F5ing the keyboard and pressing Play.

Guide to the fingering format in the console: left of the colon is the harmonic change. For example, if it asks for 5/9, you can play 5/6 * 2/3. Right of the colon are the notes to play; dashes are for simultaneous notes. Notes starting with N can't be played.

The keyboard has guard rails that prevent the fundamental frequency from going too low or too high, snapping it back to the center. This helps people hitting random keys. However, they can get in the way if you know what you're doing. You can override these guard rails by running this in the F12 console: adjust_harmony = function(i){base_frequency *= i;}

My current workflow for converting songs to keyboard format is this:
1. Try to google the song for sheet music in a computer-friendly format. No free sources will be found.
2. Try to acquire a decent midi. It won't exist, or it will have copyright.
3. IMSLP has sheet music.
4. Feed the sheet music to PhotoScore, which is very inaccurate. Correct the many mistakes by hand.
5. In Sibelius, export as midi.
6. Convert midi to the clipboard format without finding the actual just intonation frequencies. Just leave them in twelve-tone as "p15" and such.
7. Feed the approximate pitches into my music generator, which finds the right frequency by looking at harmony. Since notes are fed one by one with no structural context, this produces a lot of errors, which I again correct by hand.
8. The previous step produces music in just intonation format. Play the music, which will produce the fingering.