Hi Chris,
Well, I don't know without looking at the code, and that will be a lot easier if you let me know exactly which error you are seeing.
If you're doing full Compliance checks, then the error is reasonable, because License determination can only work if you have a Material in the item. But my hunch is that, if you're only working at the header level (SPL and Embargo checking), the developers might have been smart enough to realise that the absence of a Material wouldn't make any difference. In that case, it would be quite simple to configure an alternative Item Category to carry out a reduced set of checks, to overcome the problem for your indirect purchases.
Regards,
Dave