Supreme Court Justices are Federal employees. Congress could require them to disclose gifts and income as part of their employment agreement, with consequences for those who do not comply.
Americans deserve to know who owns their Supreme Court Justices, Congressmen, Senators, President, and Vice-President.
If a GS-203-05 Federal Data Transcription Clerk has to disclose such things, certainly the muckity mucks should be held to the same standard.