You should be able to use the 1999 app for Macintosh with your own 1999 server. Here's how.
1. Open the app.
2. If you're signed on, sign off.
4. Click on the Console tab.
5. Enter this one-line script, replacing the address of the server with the address of your server, and press Return:
localStorage.urlTwitterServer = "http://my1999server.org:1999/"
6. Quit the app and re-launch.
Now when you log on you should be logging on to your server.
If you want to return to my.1999.io type this one-line script into the console:
If you have problems, please report on the 1999-server list.