Answer
Inside a custom field's own select statement, reference another field on the same current record using the v. prefix (e.g. v.CF$_OTHER_FIELD) rather than a plain bind-variable-style colon prefix — v refers to the view row currently being evaluated. Using the v. form for the argument, rather than treating it like an ordinary bind parameter, is what makes the cross-reference to the current record's own field value actually resolve correctly.