Brian,
While you don't have access to LIKP/ LIPS, you should have access to structures XLIPS and XLIKP via field symbols. So, you can't get to XLIPS in the routine, you should be able to get to (SAPMV50A) XLIPS (put it in as a variable in debugging with the parenthesis). Put a break point at the beginning of the routine to see if these internal structures can work for you. XLIPS-VGBEL should have the order # populated even at delivery create (before the database commit occurs). As you are just reading this value and not actually updating anything within the structure, you should be okay. I would suggest reading SAP note 178328 for more information on X and Y structures and how to correctly use them.
Krishna