I personally think the EXT is harder to make in customs than Phoenix. let me elaborate...
EXT shape currently is due to the Electrostatic drives position really close to the outlet. On a Custom model, due to different ear shapes and sizes, this position would be hard to maintain and would affect the sound.
Phoenix on the other hand is a Full BA, unless they use open chamber or Tubeless design, the shell shouldn't be part of the sound signature. because the sound follows the tube to the outlet, making no/minimum contact to the shell itself. The shell in this case is more aesthetics.
Customs with different materials...this is not end of the world, as you said, Fitear does it. JH audio does it as play day. My Layla currently is Carbon Fiber infused with silver fibers. So...you get the idea. if the company wants...they can do in this case.
But...these comments are based on engineering analysis. VE should be the best one to confirm them.
I think they won't have a Custom design just because they were not meant to it. like other brands have designs that are uni only. Helps keeping customers happy as they have an uni to join the brand and helps the brand streamline the production to support demand.
my 2 cents... (a bit longer than a 2 cents...sorry)