Sorry, using the word synthetic made it sound more technical than it really is. It's a regular iCloud account. I just said "synthetic" to get across the point that the account is basically meaningless to them, they don't even know they have it, and it's confusing to even bring it up with them. The fact that the username is an email address, but it's not actually how they access that email, would lead to questions/issues for me personally. So I just set up single-purpose aliases in fastmail and own the accounts fully.