Dave,
I was not able to exactly implement the note you mentioned above, but while trying to download the note SAP suggested me the following notes and was able to implement them:
1924671, 1928576.
Even with this though, the HTS did not get populated at the Declaration line item level.
Out of curiosity when I tried creating the Declaration within GTS itself for the exact same data that I was trying while initiating from ECC, the HTS codes were populated at the Declaration lines.
Do you have any clue why this could be the case or even if you know the program/FM that populates the HTS codes at Declaration line, I can check with the ABAPer to find how SAP treats the Declaration getting generated in GTS different from the one that gets created w/ref to Pro-Forma from ECC.
Thanks,
Prashant.