Global.asa Session_OnEnd

I have an ASP application that is a electronic purchase orders system. My
problem is that only 3 people can authorise a purchase order. When a
purchase order is selected I update the table to show that the field is
locked. If they complete the form then the database is updated and the lock
is removed, all though at this stage the record moves into another part of
the system and the lock becomes redundant.
If however they dont complete the form the record is left in a permanent
locked state, it could be that they decided to look at the record then
navigated away or closed the browser.
So i thought maybe i could stick some code that updates the database into
the session_OnEnd in the global.asa, so after 20 minutes when the session has
ended the lock will free. I don't however have any experience with this and
don't even know if this will work. The code won't be a problem its just
whether this process will actually work.
I am thinking that I code put the order_id into a session variable then when
the session variable ends i run a sub in the .asa file that gets the session
variable and updates the table.