Just when I thought this CEPT thing was simple and easy, I came across this thread:
This caused me to dig deeper into the CEPT documentation. In particular, it seems that this doc:
ECO Domain Redirect lists the proper prefix for Switzerland as HB9.
And this document says that the HB3 prefix should be used for “Novice” class licenses (includes US licensees with a General Class license).
So my conclusion is that as a US Amateur Extra class licensee, I should sign HB9/K0NR. Joyce as a US General Class licensee should use HB3/K0JJW.