Context Root Masking/URL Rewrite and pool selection

You should only need to change the path in the request. You can do this with HTTP::path instead of a stream filter (which would modify the request payload):

 
  Prepend /contextroot to the original path 
 HTTP::path "/contextroot[HTTP::path]" 
 

For the response, are you sure non-200 responses won't ever contain the /contextroot? You should also explicitly disable the stream filter if the condition(s) aren't met:

 
 when HTTP_RESPONSE { 
    if {[[HTTP::status] == 200] and [[HTTP::header value "Content-Type"] contains text]} { 
       STREAM::expression "@/contextroot@@" 
       STREAM::enable 
    } else { 
        Disable the stream filter 
       STREAM::disable 
    } 
 } 
 

Also, you can use variables in a stream expression as long as you wrap the expression with double quotes (like you did) instead of curly braces.

Aaron