There isn't a way to require a context currently. Your precondition setup seems like the easiest route. If you are running command line mode, you could create a liquibase.properties file that defines a contexts property which will be the default unless it is overridden. Otherwise you could create a subclass of however you run liquibase (Ant LiquibaseTask class, command line Main class, etc) which adds in a context if none is specified.
Nathan