I fly from DCA to LGA on a shuttle flight on Friday evening quite often and then catch a late night flight out of JFK to Europe or South America all of the time (especially in the summer). With carry-on luggage, I have missed my connection just once since 1999. If there are LGA-related flight delays in the summer time, then they are generally morning and afternoon problems which have cascaded. (Rarely are the incoming LGA delays an evening problem after 8.30 p.m.).

The number of times I've left DCA at 8.00 p.m. or 8.30 p.m. to catch a c. 10.30-11 p.m. flight out of JFK cannot even be recalled. I've missed one such flight in the past 6 years and that was because of a lack of yellow cabs at the terminal in LGA and surprisingly a shortage of unauthorized transport hawkers.

If ORD-LGA is backed up, it'll more likely be a problem with ORD (thunderstorms especially) and not LGA -- especially at that hour. ORD generally doesn't have many thunderstorms after 5 p.m., but if there is a back-up, AA may well try to re-accomodate you on the ORD-LHR flight. What I would do, on the day of travel, is look up AA's flight status for ORD-LGA and call it that way. If there seems to be a serious issue, head to the airport a bit earlier so you can get reaccomodated on the earlier ORD-LHR flight or make the ORD-LGA flight. It being a Friday afternoon/evening in the summer, the biggest problem will be is that those flights will be near full (if not full).