If you ask the CM at the entrance of Royal Hall if they know when might be a good time to get in line to catch Jasmine a lot of CMs will at least check their schedules and give you a hint. Put it as neutral as you can and it usually works.
I agree the surprise is cool, but when you are hunting for a very special person it can get annoying. I was trying to find Rosetta one year and was in line at Pixie Hollow so often that in the end CMs told me to use the wheelchair ramp to skip the line and let me through when she finally showed up.