Rewrite host with an iRule?

You aren't stuck. You can do something like the following:

when HTTP_REQUEST {
   Don't allow data to be chunked so we can correctly
   replace payload below
  if { [HTTP::version] eq "1.1" } {
      if { [HTTP::header is_keepalive] } {
         HTTP::header replace "Connection" "Keep-Alive"
      }
      HTTP::version "1.0"
  }
   change host header
  if { [HTTP::host] eq "apple.fruit.com" } {
    HTTP::header replace "Host" "peanut.bean.com"
  }
}
when HTTP_RESPONSE {
    collect response data
   if { [HTTP::header exists "Content-Length"] } {
      set content_length [HTTP::header "Content-Length"]
   } else {
      set content_length 4294967295
   }
   if { $content_length > 0 } {
      HTTP::collect $content_length
   }
}
when HTTP_RESPONSE_DATA {
  set find "peanut.bean.com"
  set replace "apple.fruit.com"
  set offset 0
  set diff [expr [string length $replace] - [string length $find]]
   Get indices of all instances of find string in the payload
  set indices [regexp -all -inline -indices $find [HTTP::payload]]  
  foreach idx $indices {
    set start [expr [lindex $idx 0] + $offset]
    set end [expr [lindex $idx 1] + $offset]
    set len [expr {$end - $start + 1}]
     replace the instance of find with the contents of replace
    HTTP::payload replace $start $len $replace
     modify offset if the replace string is larger or smaller
     than find.
    incr offset $diff
  }
}

In the HTTP_REQUEST event, I've used the HTTP::header replace command to replace the host header. and in the HTTP_RESPONSE_DATA event, I've used a regexp call to return all start,end pairs of the peanut.bean.com string and replaced it with apple.fruit.com.

Good luck!

-Joe