Apache How to 'alias'/'locate'/'redirect' subsite to another internal server?

Apache Experts!

We have 'n' servers behind a firewall. One of these, 'Machine A', is the DMZ, and is world facing. It serves up any number of Apache Virtual Hosts already, no problem!

Now, how do we 'attach' another server - currently visible only to the internal subnet - such that its contents appear to come from one of these hosts, and become world-visible?, as in:
http //serverA/ServerB (essentially)

This simple directive within the vhost:

Doesn't work, as it requires an external viewer to be able to see the 'internal' server IP address, which of course he cannot.

Seems to me we did this some time ago... ????

Apache mod_rewrite

I know, I know. I've been studying it for years now... !

My off-site clients are getting errors:

"GET /wiki HTTP/1.1" 301 235
"GET /wiki/ HTTP/1.1" 302 231

I'm back on this tonight.

I am really, really having trouble with this. May I ask for someone's help with this?