I've updated the document about our implementation approach and would be happy to hear of any feedback or advice about it.
One thing to bring to attention was we realized how much the mod_lti module is currently gravitating around the presence of a Link. This caused some tension with the decoupled (i.e. link-less) line items, in particular as we want our initial implementation to be contained strictly as a subplugin. We think we have found a workable compromise and would love to hear if there are major concerns with the trade-offs we had to take.