Isn't it the mailbox owners responsibility to provide the post office with a key for locked mailboxes?
That was my understanding when something similar happened at our last apartment building, rendering a bank of boxes inaccessible to the mailman.
According to Section 2.1.7 of
this page, maintenance of mailboxes are entirely the owners responsibility. I'd press the HOA hard, it may just be that they need to provide a new key to the post office.