Is it even the case that you are legally protected when you inform you coworkers that they have constitutional rights? I would have assumed only informing them of workplace rights specifically would be protected.
Only if it's a concerted effort to improve the workplace and even then it depends on how it is accomplished.
The message merely stated that employees have the right to organize. Without additional context, I don't see how that is a concerted effort to improve the workplace. There would have to be a "unionize to improve health coverage, or wages, etc" type message. Otherwise, I could just stand at my desk all day and tell people as they walk by that they have the right to free speech, to own a gun, and protections from unreasonable searches, etc. It's not a concerted effort to make the workplace better, so it doesn't matter if I'm telling people about their rights (not to mention I would not be doing my job while on the clock).
Using a plugin to communicate that message is likely a subversion of it's intended purpose and would be inappropriate - similar to if she were maintaining the company intranet site and posted a banner with that information. They aren't mediums for individual communication (such as email or chat apps) but rather for company-to-employee messaging and for the sole use of the company. In this case, she misused company-to-employee infrastructure outside of its scope for her own communication purposes.
