ProxyPass STREAM replacement of hostnames

I suppose you could, but then that doesn't actually exist. Browsers don't show it, but in the absence of a specified URI path, there will ALWAYS BE a forward slash / after the host name.

Just to give you an example, try this iRule on an LTM VIP:

when HTTP_REQUEST {
    log local0. [HTTP::uri]
}

When you test, don't include a URI in the browser, just the host name. The URI will be "/".

And then try this from the BIG-IP shell:

curl -v http://x.x.x.x

where x.x.x.x is the IP address of any port 80 pool member. You're going to see something like this:

GET / HTTP/1.1
Date: some date
Server: Apache...
Host: x.x.x.x
Accept: */*

Notice that even curl plants a / in the request.