не то, что хотелось бы, но у меня работает
CREATE DEFINER=`root`@`localhost` FUNCTION `convertunicode`(encoded varchar(100))
RETURNS varchar(100) CHARSET utf8mb4
BEGIN
DECLARE decoded VARCHAR(20);
SET decoded = encoded;
SET decoded = replace(decoded, "u00e1", "á");
SET decoded = replace(decoded, "u00e9", "é");
SET decoded = replace(decoded, "u00ed", "í");
SET decoded = replace(decoded, "u00f3", "ó");
SET decoded = replace(decoded, "u00fa", "ú");
SET decoded = replace(decoded, "u00c1", "Á");
SET decoded = replace(decoded, "u00c9", "É");
SET decoded = replace(decoded, "u00cd", "Í");
SET decoded = replace(decoded, "u00d3", "Ó");
SET decoded = replace(decoded, "u00da", "Ú");
SET decoded = replace(decoded, "u00f1", "ñ");
SET decoded = replace(decoded, "u00d1", "Ñ");
RETURN decoded;
END