I don't think that's a strange suggestion at all, it's a great idea, thank you. I have contemplated doing a meditation before all my practices just so I'm going into my drumming totally relaxed - hoping I can condition the relaxation response in my body/mind whenever I sit down at the kit. I could probably use visualisation as part of that.Not sure if this will help but a few minutes before your exam close your eyes and visualize yourself taking the exam. "See" yourself playing well and mentally focus on the process and your playing. It may sound weird but it works for athletes before a competition.
Trying to put it into perspective too - it's just a drum exam, it's not the most important thing in the world and whatever happens, I will survive. I think I just really want to enjoy it. Nerves are so dumb - they don't make sense (only to the ego!!).
Thanks for your suggestion, I appreciate it. I'll definitely be leaving the vomit out of the visualisation.