Issue with ALIMSKSOH entry point in TRTVENISOC

I am trying to populate a field on an intersite sales order when created automatically from an intersite purchase order.

I have identified the ALIMSKSOH entry point in TRTVENISOC which should allow me to do this. Unfortunately the field that I wish to populate is on a custom tab screen, which does not appear to be loaded as a part of the entry point (SOH0, SOH1 and SOH3 are loaded) and I can't move the field to a one of the standard screens as it is referenced by a number of other customisations over which I have no control.

Any suggestions of how to get around this limitation would be welcome.