Anonymous quizzing in Storyline

For a project I am working on, we are asking some sensitive questions such as: "Have you ever taken recreational drugs?".

It is important that the client knows all their staff have completed these storylines and to identify who hasn't completed it, but importantly no one should be able to identify who gave the individual answers.

My idea is to host the storyline on our server, give organisations a pass for their staff to access it (so they never need to log in with a personal account or enter identifyable details). The user does the quizzes and the data is collected on our servers - anonymised and encrypted. When the storyline is finished, the user is given a unique key (we will make sure this is not linked to any of the quiz answers). They then enter that key in the organisation's IT system that checks to see if it is valid. If valid, it tells the organisation that they completed the storyline. 

Has anyone done anything similar or have any thoughts?