Alastair, The only way to accomplish this without a customization is through direct SQL. The script below will allow you to set an item message to items given an item message title and a category code:
DECLARE @ItemMessageTitle nvarchar(30)
DECLARE @CategoryCode nvarchar(17)
/*Set variables here */
SET @ItemMessageTitle = '' --Set the ItemMessage title to be used
SET @CategoryCode = '' --Set the Category Code to be used
UPDATE ITEM
SET Item.MessageID = (
SELECT ID
FROM ItemMessage
WHERE Title = @ItemMessageTitle
)
FROM Item
LEFT JOIN Category
ON Item.CategoryID = Category.ID
WHERE Category.Code = @CategoryCode
This script is provided as is with no implied or expressed warranty. Please make sure you always backup your database.