Dave, I'm not a national champion -- just a lowly magazine publisher.
Based upon what Jamie said, it sounds like your class has to be a paid-up member of US Sailing in order to have your national champion participate in the Alter Cup.
If that is the case, seems like they would go down through the US Sailing directory and pick out the classes that are paid up and send them notices that their National Champion is eligible to participate in the Alter Cup.
If those classes decide they don't want to participate, THEN you go back to including more people from the classes that DO want to participate (and are paid up members of US Sailing, of course).
And then, of course, you get the debate about which of those paid-up classes should get two people instead of only one.