iRule to check for strings in xml

Well technically you're code might look something like this:

when HTTP_REQUEST {    
    if { [HTTP::header exists Content-length] }        
        HTTP::collect [HTTP::header Content-Length]
    } else {        
        drop
    }
}    
when HTTP_REQUEST_DATA {    
    if { ( [HTTP::payload] contains "MessageID" ) and ( [HTTP::payload] contains "MachineName" ) } {        
         good request
    } else {        
        drop
    }
}

The above should very specifically limit access to only requests that have a Content-Length header and contain these two values in the payload. But...

The example request you give is part of a 100-continue, which usually implies another request is coming with the rest of the payload. Are you certain, from captures, that every request has these two values? Will these values be in the very first requests?