Immutable struct with AA init problem

[Uranuz]

As far as I understand in current implementation of Dlang it's 
not possible to write:

immutable(EnumFormat) magicCreatures = 
immutable(EnumFormat)([1:"goblin", 2:"ork", 3:"elf", 7:"dwarf"], 
false);

It's because AA literal isn't immutable. But is there some method 
similar to assumeUnique() for AA? Using static this() to define 
some constants looks awkward as I believe.