I agree that Dumbledore doesn't seem able to get Lucius to delay the trial. But I suggest that Harry could suggest an agreement to delay the (irreversible part of the) punishment.

Since the only fixed rule of the wizards' council is there are no fixed rules, such an agreement could be made. "She's guilty, and we'll house-elf her in a week." Since Lucius' priority is protecting Draco, genuine proof that someone else tried to killed Draco would be plenty of reason to release Hermione -- in exchange for Harry/Voldemort's help against the villain.

From Harry's point of view, especially, why should Lucius exact a high price to postpone the house-elfing of Hermione by a week or a month, as long as he has the formal verdict and the young villain is under secure observation somewhere?