Is it possible to get a list of all mailboxes (email addresses) with data on the archive server?

You can run the following SQL script ...

Use ArchiveManager
select ma.MailBoxID, ma.name, ma.enablestoremanager, ll.loginname, ma.OwnerLoginID, ll.emailaddress,SUM(CAST(ME.Size AS BIGINT)) / 1024 / 1024 AS MailboxSizeMB
FROM MailBox AS MA INNER JOIN
MailBoxMessage AS MM ON MA.MailBoxID = MM.MailBoxID INNER JOIN
Message AS ME ON MM.MessageID = ME.MessageID, Login AS LL
where ma.OwnerLoginID=ll.LoginID
group by ma.ownerloginid, ll.LoginID, ma.mailboxid,ll.loginname,ma.name, ll.EmailAddress, ma.EnableStoreManager