[Zope] how to override SERVER_URL?

I need to override SERVER_URL so I can trick Zope in to giving me the correct base href. I can't figure out how. I tried exporting it as an environment variable, and putting it in the http headers - but neither worked. Can anyone help? I'm runnig ZServer without apache. FWIW, the reason I need to do this is I'm trying to write a simple load balancing proxy server with failover support that works at the http level and not at the tcp level for use with ZEO. The problem is the base href returned to the client points to the actual zope server, not the proxy server. Thanks, ^Roman