Ah...
Well, where do you plan to 'intercept' the flow ? Before the file is inserted into db directly from the temp dir or just later or selectable by Admin ?
I mean, the attachment could be easly go into database and the Admin could have an option - near the attachment moderating - to save selectable files from table to directories; this way, the link on posts shouldn't be affected at all, as its target is attachment.php with a query_string.... and only this php should do the entire work.
This was my idea when started thinkin' about, but absolutely I'm not able to throw out the db data field to a file....

irate: