Hi Ben, I'm moving out of the LMS space in the coming week but I went ahead and raised an improvement request (we're using 3.5 currently) so it can at least be considered to be included in to the core APIs. A mandatory parameter that can't be obtained is a bit annoying so would be great housekeeping for it to be sorted.
Create an account and vote/watch here: https://tracker.moodle.org/browse/MDL-70582
Create an account and vote/watch here: https://tracker.moodle.org/browse/MDL-70582