Can they make a Vader with an extended scale ? no, and he said that the B string is plenty tight. I'll have to see for myself because Carvin puts some pretty cheesy strings on their basses.
They use Dunlop Superbrights, which are some of the best strings available. I will say that they probably use stock packs and therefore ship their basses with a .120 or .125. The .120 is fine. I haven't tried the .125. You can order single .130's, which is what I do. the .130 is quite good. You can send them a .130 to install on your bass, at the factory.
They list a 12" radius, as an option. Usually 4 stringers from Carvin, also have 10" as an option. It seems that the Vader may not. Maybe something to do with the hardware piece which goes on the end of the neck. Maybe they don't make them for 10".I still need to ask them if the Vader neck is available in another radii - something a little bit rounder (9" or so for example)
I'd like to know if the Vader will accept flatwound or heavy gauge strings. I'll need to see it closely in person.
You can order these fret-less, so yes, flatwounds will work. The bridge and headpiece are designed so that you can use regular bass strings. You do not have to have double ball end strings or some sort of special headless string.
As far as gauges go, I would assume that most anything which would work stringing through body on an Hipshot A bridge, would work here. So not TOO heavy.