I had a look and I actually can't find any cases that address an employee sharing their salary with third parties, so I might be wrong (or just bad at looking for relevant legal cases). However I can find numerous cites where people mention off-hand that 'of course this doesn't mean employees can share salaries outside the company' and similar - http://www.velaw.com/uploadedFiles/VEsite/Resources/Employer...

I would suggest anyone attempting to claim NLRA protection actually read the document, particularly clauses like the exclusion of protection for supervisors discussing salaries.