The IMS Gradebook services (formerly known as Outcomes Management 2) are currently in Candidate Final Draft (pending completion of sample implementations and certification tests before it becomes final and public). These services are designed to provide LTI tool providers with more control over the management of line items (columns) and results (cells) in the gradebook of an LTI tool consumer. Cengage is supporting an implementation of these services for Moodle as described on the project page at https://docs.moodle.org/dev/LTI_Gradebook_Services. Comments and feedback welcome.
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.