Change the standard client to another port:
Changing default page Advanced Client to Basic Client - ZimbraWiki You can then make url pointers to it (other links and/or your dns) and have the zimbra tomcat port 80 landing page redirect if you want.
This is already coming in v5:
Bug 14181 - Force Basic HTML Client for Users based on COS
Check out the related blog post yesterday:
/blog/archives/2007/08/zimbra_standard_client_1.html