MySQL - using result of Procedure in Trigger to Insert in new table
I believe I have found the solution, and that is to convert the Procedure to a Function.

DELIMITER $$
    CREATE FUNCTION fn_get_item_type_id( item_name varchar(45) )
    RETURNS INT DETERMINISTIC
        RETURN (SELECT 
            CASE
                WHEN LEFT(`item_name`,6)  = 'abcdef' THEN (SELECT t.`id` FROM `item_type` t WHERE t.`item_type_name` = 'type1')
            ELSE 0
            END AS `item_type_id`);
    $$
DELIMITER ;


From there I can simply Select it into a variable in the Trigger.