Erm, I'm not entirely certain how DW is doing this, but wouldn't it be that one has a private/personal acct (not for making at-work statements as staff) and then an account that's the official staff/I-work-here account? In that way, if someone leaves DW as staff-member, that account simply goes inactive, as it would for any other account. (Your comments don't disappear just because the account's closed, right?)

The use of a personal account for work-stuff and personal-stuff is what leads to a) people being confused as to when one is speaking ex cathedra, and b) situations like this hypothetical, of ex/former staff. If staff have professional staff-only accounts, wouldn't that avoid this entire question?