(posting a solved issue as a guide for any admins possibly running into this same situation)

Summary of Problem: zmamavisdctl not running, mail not being delivered
Summary of Solution: delete stale amavisd.pid, restart zimbra, manually restart postfix

After a power outage and a reboot of our mail server, mail was not being delivered to user mailboxes. Mail was still going out however, but a look at the Admin Console showed mail piling up in the defered queue. Restarting Zimbra showed this:

        antispam                Stopped
                zmamavisdctl is not running
        antivirus               Stopped
                zmamavisdctl is not running
and restarting it via the command line only gave a "Starting amavisd...failed." message.

The /var/log/zimbra.log file showed that there was a stale amavisd.pid file and that was preventing amavisd from starting. I deleted the old pid file and amavisd started.

This didn't get mail flowing however. Postfix was not delivering mail - possibly because of the amavisd inability to start when postfix was spawned - so a "postfix reload" as the zimbra user got the mail to process out of the queue and deliver to user mailboxes.