Would walking in the grass around the suitcase work? As it's Diamond he's restarting.
EDIT: Just realised that the rival took you through the grass, could that advance the frame?
I don't know if that would work or not. I haven't reset on diamond in a while, so I don't know if you can walk in the grass or not. I think it wouldn't work because there is no chance to encounter a pokemon, unless they programmed it to where there it is treated like a repel is up and still able to walk in the grass. You can test it if you want. Save in front of the case and take it immediately, then get your seed later based on the pokemon, then do the same thing but take an extra turn in the grass and get the pokemon. If the first one is on frame X, and the second one is on the additional frames from walking in the grass, then it is possible most likely.
Austin - Don't post questions like that. It is irrelevant. The maximum frame should be what you are willing to do. Some people have gone for frames in 10000 before I bet, while others won't go over 2000. This is a bad question and changes for everyone. It all depends on how much time you have and if you want a flawless pokemon or not.