I ran into the same problem. In simple terms, you can't.
First, you must use a reverse proxy to translate the requests, not just port numbers (so a firewall redirect is out, it would break links)
To accomplish this with Zones (which I assume you're using) the extxxx.xxx.info would have to re-write its links to xxx.xxx.info (presuming you're using a reverse proxy to translate xxx.xxx.info to extxxx.xxx.info on the inside so that WSS knows which zone the user is in), but that's not an option, even with Alternate Access Mappings. You will get errors 'the public url already exists' or 'cannot remove the default zone primary url'.
It would be a nice hack to just tell Sharepoint to re-write a public url to something arbitrary, regardless of the private url or existance of other private/public urls, but I suspect this would break something else. I was excited about alternatte mappings, till i realized that this is not an option.
Your other option is to become very familiar with a regular-expression filtering reverse proxy like mod_proxy for Apache. This will break some stuff right off the bat (like searching).
Sorry, but that's the best I can do.