The BES is something that is created and Maintained by Research in Motion. Zimbra didn't create it and, and has no control over the architecture BES requires. Zimbra has no option here, they have to work within those boundaries.
We have a BES setup in our environment, so I can't speak to this directly, but have you looked at
AstraSync? It is supposed to add ActiveSync capabilities to BlackBerry phones. Since Zimbra's mobile add-on handles ActiveSync connections, that should work. Zimbra is specifically listed on AstraSync's page.