How to replace URL before forwarding

I hope someone can help me with this issue:

I need to have a client make a request thru Privoxy to a
non-existent domain, then for Viproxy to remove some
characters to make it valid, and do the opposite on
response. For example:

-User requests http://www.zzSPzz.realserver.com/

-Proxy intercepts request and removes zzSPzz. before
forwarding the request so the request is sent to the
internet as http://www.realserver.com/

-When the request comes back to Viproxy, it adds
zzSPzz. back to the URL so that it is returned to the
client as the original request, with the invalid domain
name.

This would happen for any requests to that particular
domain.

It may seem like a strange request, but we need to do
this in order to index all versions of pages that have the
same URL but are served in different languages
according to the user logged on. I tried using a filter,
but the filter is applied on the way back, not on the
request out from Privoxy. Please advise.

Thanks,
Andres