The first place to check would be with your pass bureau. The reason you want to call them first is to find out whether your company issues paper tickets for non rev or e-tickets. If they still issue paper tickets (DL does still accept them, right?) then you will have to check in with a person at the counter. This also makes a difference if you need to change your routing on the fly. You might also want to verify with the pass bureau whether your agreement is a ZED agreement or not and what the level is. Having this information helps you look at different airlines that offer similar routing if DL turns out to be full.