I've always believed that doling out the freebies in advance creates more ill than goodwill in the long-term. DL has pretty decent software, does a good job on the RM/IM front and really is left to account for last minute misconnects and other IRROPS.

Best to have the GA run the UG list just before T-15, and then call the UG's as the clock strikes T-15. At that point, you're in or you're not. People who paid for F or are otherwise booked into paid FC don't get bounced and those who get the UG don't get DG other than in extraordinary circumstances, and the flight still pushes as scheduled.

This also becomes another competetive advantage of engaging in what some here derisively refer to as FCM and what people in the real world call selling your product. The RM/IM system will price F UG's at what it believes the market will pay at any given point and passengers who want to sit up front will either pay that (if they didn't book into F from the git go) or chance the T-14 UG.