As excited as I am for this jam, I'm worried about how much I can actually get done in an hour. I have a dialogue system I made for a previous game--could I port it to my one-hour game? Or would that not be allowed? And what about external assets, like the All-In-One 2D Shader? Those are really the only two external things I would use, and I can try to remake what I need of them within the hour, but I'd love to save a bit more time. Thanks so much :))))

It seems like none of this is possible.

To reuse whole systems like that is not really the spirit of the jam, unless you're talking about a game-engine-like tool.  Games that are dead simple, and silly is kind of what we expect.