Hi Dean,
From a rather quick look at the program code, it doesn't look promising...
It starts well enough: the feeder system mapping correctly identifies third-party sales transactions and determines the correct departure and destination countries for the data transfer to GTS. But in the re-mapping, the header countries (CTYDP, CTYAR) get copied from the first item and are then used for the Compliance checks.
As you know, there are individual checks for each item in the order, but the relevant country - and therefore the Legal Regulation(s) - is determined from the header data.
Your best bet, therefore, could be to create (and check) individual Sales Orders and have them combined either for Delivery or for Billing.
Hope that helps.
Regards,
Dave