• The Dementor is literally death. The "sword that has slain a woman and rope that has hanged a man" ritual will almost certainly summon one, but that's known Dark, and thus probably not something that can be used in the middle of a Wizengamot proceeding. And other than altering the punishment, how would this help? Even killing the Dementor outright will just make them mildly annoyed.
• Dumbledore did (plausibly) burn Narcissa alive, and Potter saying so openly might be enough to swing something. It'd be unlikely to turn out well - Dumbledore would of course deny it, Potter's alliance would instantly be sundered, and unless Dumbledore wound up in jail, it wouldn't save Hermione. But, it might be tried.
• The scarred man is likely Jugson, not Greyback. Isn't Greyback in Azkaban right now? Not a solution, but it should be noted.
• If he's learned Avada Kedavra, there's always the option of blinding everyone with a super-Patronus and then committing mass murder until your side has a majority. Somehow, I don't see that one happening.
• Snape and/or Quirrell(or someone else - Padma Patil would be a funny choice) comes to the rescue. Vanishingly unlikely, and hardly in keeping with the message of the story, but not strictly impossible.
• Hermione figures out the super-Patronus, with Harry's prompting. This one is actually the least crazy of the lot, I think - the super-Patronus works on the principle of love for all human life. Someone who casts it ought to be damn near incapable of murder, and if the principle could be explained to the Wizengamot without ruining everything, the fact that Hermione managed it would actually constitute exculpatory evidence. It likely wouldn't be believed, but it's closer to possible than most of the others.

As I said below though, these plans all share one common feature - they suck. I can't think of one that isn't either vanishingly unlikely or obviously stupid, and too stupid at that to be used even by a despairing child trying to save his girlfriend from a fate asymptotically approaching death.