Cache based on SOAP XML Envelope content

A developer of ours came up with a brilliant idea, which I'm trying to implement now. What if we make an md5 out of the content and cache using that?

I wrote the iRule below but despite the devcentral page saying that the CACHE method can be used in the HTTP_REQUEST_DATA it seems like that's not the case.

Thankful for any input!

when HTTP_REQUEST {
  if {[HTTP::method] eq "POST"}{
     Trigger collection for up to 1MB of data
    if {[HTTP::header "Content-Length"] ne "" && [HTTP::header "Content-Length"] <= 524288}{
      set content_length [HTTP::header "Content-Length"]
    } else {
        set content_length 524288
    }
     Check if $content_length is not set to 0
    if { $content_length > 0} {
      HTTP::collect $content_length
    }

  }
}
when HTTP_REQUEST_DATA {
    Collect the data needed
    set payload [HTTP::payload]

    Make the md5sum
    set md5 [md5 payload]

    Store the uri for later use (the service might not be able to accept a uri)
    set uri [HTTP::uri]

    Set the uri to current uri+md5
    HTTP::uri "$uri?$md5"

    Cache the uri to current uri+md5
    CACHE::uri "$uri?$md5"

}
when HTTP_RESPONSE {
    For troubleshooting
    HTTP::header replace X-Via "F5 BigIP (DMZ)"
}
when CACHE_RESPONSE {
    For troubleshooting
    CACHE::header insert X-CacheStatus "Hit"
    CACHE::header insert X-CacheAge [CACHE::age]
}