you can use the API on the 400vx. we do so very heavily on ours (add/edit/delete domains, a/e/d filters, a/e/d white & blacklists etc).
at the moment, it seems you can't pull a list of users per domain, only the global one. but for stats, i do this just like this -> i pull the global list, and then process it to narrow it down to specific domains (simple loop and if condition if listlast(@) matches my desired domain. that works very well.
just be informed that not all inbound email addresses have a user created for it - only if the user does it manually or there is quarantined files for it. we do per email billing on our side and i use the syslog to dump all log data into a database and then i filter by recipients and domain, counting up all recipients having received at least 5 emails for the past month (in an attempt to grab really used email addresses and discard seldom used aliases).