I imagine that this is something that could not be done with the assistance of an airline. The only unique identifiers to determine the plane would be the N number and the internal United number which is usually on the front side of the plane, sometimes below the ETOPS identifier. You have to keep in mind that they could always change the actual plane at the last minute to the mechanical reasons, etc and this would have to be updated.

Another cause is sometimes equipment gets changed based on weather. This did happen on 2/4 when they were using a 747-400 to fly between LAX-SFO since basically every shuttle flight was cancelled.