Is it possible to read the user in a user defined fuction in a PI mapping?

I am running PI 7.0. I have a service call to the HTTP adapter on my Integration Engine to initiate a message. I would like to capture the username used to connect to the HTTP service when this message was called.

The call is initiated with the endpoint http://<myhost>:<http port>/sap/xi/engine?type=entry&sap-user=THISUSER&sap-password=MyPassword&version=3.0&Sender.Service=MyBServ&Interface=MyNamespace^MyInterface

When I look at the message in the RWB, I can see that the username is captured as part of the SOAP Header, in the RunTime area of the input message,

<!-- Inbound Message -->

<SAP:RunTime>

<SAP:Date>20101216</SAP:Date>

<SAP:Time>102057</SAP:Time>

<SAP:Host>myhost</SAP:Host>

<SAP:SystemId>xxx</SAP:SystemId>

<SAP:SystemNr>xx</SAP:SystemNr>

<SAP:OS>OS400</SAP:OS>

<SAP:DB>DB400</SAP:DB>

<SAP:Language/>

<SAP:ProcStatus>000</SAP:ProcStatus>

<SAP:AdapterStatus>000</SAP:AdapterStatus>

<SAP:User>THISUSER</SAP:User>

...

Is there a way that I can return this username in a User Defined Function (UDF)?