Two reasons you cannot check a bag if you want to pull this:

1. If under the pilot program, your bag will be sent on to YYZ and you will not see or touch it at IAD unless CBP pulls it on a random or selective basis.
2, If under the standard program, you will collect your bag at the Midfield terminal, process it through CBP and then go through a checkpoint. If it is a full-sized bag, you won't be able to do so.

All of this is easy with carry-on. Simply go to either International Arrivals or Midfield, go through CBP and either walk out of the airport (IAB) or hop on the AeroTrain to the Main Terminal and walk out of the airport (after a security check).