Setting a message element value inside orchestration.

So I've been looking around at promoted properties and distinguished fields and I'm not sure how to approach my problem.... Basically all I need is for a 'MessageIdentifier' property to be set to the current message's MessageId within my orchestration.

Here is my code for constructing the message.  The FtpTransactionMessage is of XmlDocument type so that I can use custom maps and pipelines at runtime, without defining a construct for every specific type of message I could encounter.

FtpResult = "0";
FtpMapType = System.Type.GetType(ConfigurationClientConfig.FtpConfiguration.Map);
FtpToEmsPort(Microsoft.XLANGs.BaseTypes.Address) = "ftp://" + ConfigurationClientConfig.FtpConfiguration.RemoteFQDN + "/" + ConfigurationClientConfig.FtpConfiguration.Filename;

construct FtpTransactionMessage 
{
    transform (FtpTransactionMessage) = FtpMapType(MessageIn);
    FtpTransactionMessage(FTP.UserName) = ConfigurationClientConfig.FtpConfiguration.Username;
    FtpTransactionMessage(FTP.Password) = ConfigurationClientConfig.FtpConfiguration.Password;
    FtpTransactionMessage(FTP.RepresentationType) = "ASCII";

    FtpSendPipeline = new Microsoft.XLANGs.Pipeline.SendPipelineInputMessages();
    FtpSendPipeline.Add(FtpTransactionMessage);

    Microsoft.XLANGs.Pipeline.XLANGPipelineManager.ExecuteSendPipeline(System.Type.GetType(ConfigurationClientConfig.FtpConfiguration.Pipeline),FtpSendPipeline,FtpTransactionMessage);
}

The input message, 'MessageIn', as well as 'FtpTransactionMessage' are both XmlDocument types in the orchestration.  MessageIn gets converted to FtpTransactionMessage (using XLANG) which is a colon delimited flat-file which gets FTP'd over to some server for processing.

At some point in this transformation I would like to include the orchestration MessageId as the first line in the flat-file.  I would like to stay away from a custom pipeline component, if possible.

Any help would be appreciated,

Chris