T-SQL (SS2K8) :: Always Set A Field To NULL

I would like to force a column to be null all the time. I cannot alter the table structure or alter the code that inserts data.

create table dbo.tblCustomer
(
CID int IDENTITY(1,1) not null,
Fnamevarchar(20) not null,
Lnamevarchar(20) not null,
Extravarchar(20) null

[Code] ....

So when this is executed the field Extra is always NULL

INSERT INTO tblCustomer (Fname, Lname, Extra)
VALUES ('bob', 'smith', 'ignore'), ('jane', 'doe', 'empty')
update dbo.tblCustomer set Extra = 'something'

If I've understood After triggers correctly the data will be written and the trigger will fire and overwrite. To avoid 2 writes

I could create an INSTEAD OF trigger

CREATE TRIGGER TR_I_Customer
ON tblCustomer
INSTEAD OF INSERT AS
BEGIN
SET NOCOUNT ON
INSERT INTO tblCustomer
(Fname, Lname, Extra)
SELECT Fname, Lname, NULL
FROM Inserted
END

This will not write the "extra" field twice. However if a new Nullable column were added; it would be v.easy to forget to update the Instead Of trigger. Everything would still work OK but I would be effectively ignoring the new column as well.

What I would like in the instead of trigger is do something like this...

UPDATE INSERTED SET Extra=NULL

Continue with insert without supplying column /value list.

What would be the best way of achieving this, trigger is the only way I could think of?