How to rewrite HTTP payload using Irule

Well, just a hunch here, but guessing you could accomplish this with a stream iRule. Apply a blank stream profile to the VIP and this iRule:

when HTTP_REQUEST {    
     disable server side compression         
    HTTP::header remove Accept-Encoding        

     disable stream for requests         
    STREAM::disable
}    
when HTTP_RESPONSE {    
     only do this for text-based responses        
    if { [HTTP::header Content-Type] contains "text" } {        
         establish the stream expression             
        STREAM::expression {@https%3A%2F%2Fabc.com@http://xyz.com:8080@}

         enable stream processing            
        STREAM::enable
    }
}    

You may actually need to do this:

{@https%3A%2F%2Fabc.com@http%3A%2F%2Fxyz.com%3A8080@}