Dude... I know what a load cell is and does. When you said "...If the curve is calculated using a load cell, then there's no need for a load cell there" it appears you are saying Logitech's use of a progressive curve is to emulate a load cell. If this is what you are saying... I disagree. A progressive curve is effectively a way of softening the system's response to your input right up until it HAS to amplify your response to get back on track to the 100% signal. That means it is effectively a moving goal post. I don't think that is an effective way to model a load cell out of a potentiometer. Actually I think that would be one of the worst ways to do that.
Mmm.. Not exactly what I'm saying but close.
If they decided that this brake mod that they are using is what feels good to them for replicating a hydraulic feel, then the way for it to behave like a hydraulic brake, is to calculate the exact curve for this exact material combination.
Meaning that they should have connected a load cell and the potentiometer at the same time. At any given position, the pot will give a value(axis x) as well as the load cell(axis y) This will give the curve of this specific setup.
So the curve is not trying to mimic anything, it's correcting a value by a sensor with the memory of another sensor. This would be brilliant in order to save money if it would be that more expensive to actually use a load cell. That is, of course, if they used the aforementioned method and not a curve from their mind. (which is much more probable imho)
This does not have to do with the feel though. And unfortunately, if you change the feel, then the curve is not right at all. You'd have to "recalibrate" it..
Unfortunately I am on vacation and cannot make any diagrams to show different curvatures, depending on progressive materials.
Don't get offended here, but I think you're a bit confused and not exactly understanding how a load cell works, because you said that the conical brake replicates a load cell brake. It's not. It's one sided feeling modification, but with no ingame translation. You're confusing feel vs braking force ingame.
I would be happy to explain further if you wish, but I think I covered the subject.