Actually i have the same problem. So if you find a way please let me know (email@example.com)
I've put a workaround in using a session file which records what the user has in their cart and i tried to use onsession_end to back out the stock but you cannot do any database stuff in global.asa to what i do is maintain a user count and if it is one then i do some housekeeping and also at a certain point in the site the system checks to see if there are any old session records lying about and clears them up. I set an application to prevent more than one person doing this.
(this is the ticket site for u2 tickets with stock control)