To elaborate a bit, my suggestion is:
1. When the student submits their Page, within Mahara we create a non-editable, non-deletable copy of the page (using basically the same mechanism as the existing "Copy a Page" button). We'll call this a "Snapshot".
2. We send the URL for that Snapshot to Moodle.
I discussed some of the implementation complexities in this later comment: https://moodle.org/mod/forum/discuss.php?d=231356#p1006613
I think the best thing about this solution is that it solves the perennial problem of people not wanting to "lose" their original Page by getting it permanently locked when they submit it to an assignment. Also, by creating a "Snapshot" API, we'd be laying the groundwork for my dream of adding a Page versioning system to Mahara.
In this scenario, there would be no such thing as a "draft" submission, really. However a student could potentially go in to the assignment and change their submission to a different Snapshot. There is also nothing we need to do on the Mahara side when the submission deadline happens.
Cheers,
Aaron