How to Set a Custom User Agent String for Chromebooks in Kiosk Mode?

I'm trying to find a way to configure Chromebooks in Kiosk Mode for a school district to ensure they can only access specific resources during testing. Ideally, we want the server to recognize that these devices are in Kiosk Mode, so we can keep track of when exams are taken. I considered adding a custom query string to the start URL for the exam website, but that's only effective at the beginning of Kiosk Mode and doesn't help with ongoing checks. I'm wondering if it's possible to modify the User Agent string in Kiosk Mode. If not, I'm open to other ideas for effectively identifying systems running in Kiosk Mode.