Serverside redirects

The server-generated redirect URI is contained in a server response header, so rather than using [HTTP::uri], you can use [HTTP::header] to catch and manipulate the Location: header within the HTTP_RESPONSE event:

when HTTP_RESPONSE {
  if { [HTTP::header exists Location] }{
    set myNewRedirect 
    HTTP::header replace Location $myNewRedirect
  }
}

However, you might want to experiment with a persistence profile instead of an iRule to solve the problem.  Persistence is easy to implement.  Cookie persistence is low overhead, and would work if redirects are to the same server name.
HTH
/deb