The Lorentz transformation is a transformation between reference frames, not objects. Sometimes we can be sloppy and say things like "the Earth's reference frame", but it is important to understand that what this means is that we're picking a reference frame where the Earth is at rest. If an object moves at least as fast as light, no reference frame exists where the object is at rest, and trivially there can't be a Lorentz transformation to a non-existing frame.I understand what you mean, but I don't understand how this changes the situation. If two objects have a mutual relative constant velocity with respect to each other, the Lorentz transformation applies.
It is not a matter of agreement -- all Lorentz transformations I used use v < c, which can be readily checked since I was very explicit and showed every step in the argument. You would use a hypothetical v > c Lorentz transformation, should one existed, to describe what happens from the perspective of one of the bubble's occupants -- but that is precisely the regime where the theory is inapplicable because of high spacetime curvature (in Miguel Alcubierre's original formulation, there is no time dilation with respect to Earth for the bubble's occupants, but you'd never know that from special relativity alone). Instead, we use the Lorentz transformation to a different observer, Alice, who's moving subluminally and merely watching the bubble. Relativity says her perspective is valid and she sees the bubble move backwards in time. Either you reject the bubble, or relativity, or causality. It's airtight.The carrier for superluminal communication can be a tiny container driven by a hypothetical Alcubierre warp drive. Its velocity relative to the sender would be 7.884.000c. Since it is an object that has a velocity relative to both the sender and the receiver of the message, the Lorentz transformation applies. Alas, this transformation does not yield sensible results for v>c, so we're left in the dark. I do not agree with Markus that "we use v<c throughout".