Rhythm Lunatic

There are no servers left as far as I know, since http://stepmaniaonline.net/ doesn't even have account creation and leaderboards anymore The feature is for SMOnline servers, the source code is...

It is documented in /Docs/Devdocs/SMO_protocol-aj.txt

Authentication and registration is left to the server to implement, SMO is just a protocol.

Chrispable's fork (and my fork, being based off his) has scoreboard submission. There is no authentication or verification, it's probably meant for arcades only.