You can get nearly the same result with less trust: let the employee draft a goodbye message and have the boss (or so) forward that to the other employees.
The company letting a manager relay a message, with any censoring, is certainly better than the person having no way to get their contact info to people, and they might also say something nice for morale.
My suggestion was meant as something that's feasible even for a company that already got burned by vengeful leavers; and also something that an individual manager has an easier chance of pulling off, without having to change all of corporate policy.