From my understanding of the AGS spec's Score Publish Service, there isn't support for doing bulk grade updates. Likewise, Canvas's implementation of AGS doesn't appear to have an extension to support bulk updating of grades through LTI. For what it's worth, there is a REST API endpoint for bulk grading, but that would require generating a second set of credentials for the REST API and would sidestep the interoperability benefits you get with LTI.
