Non-traditional key signature with double sharps/flats?

[Brian Voyer]

Hi everyone,
I'm currently working with a synthetic scale with a doubly augmented second,
and I want to show the 
synthetic key signature in the score. 
The problem is, starting on C, I have the next note a doubly 
augmented second up, a Dx. 
I'm looking for something like the standard syntax for defining a scale, 
but I need the ability to have double sharps and flats.
synthetic = #`((0 . ,NATURAL) (1 . ,SHARPSHARP) (2 . ,SHARP)
    (3 . ,NATURAL) (4 . ,FLAT) (5 . ,FLAT) (6 . ,FLATFLAT))
Sadly, this doesn't work. Does anyone have any ideas?
Best,
Brian