I did manage to do something like this:
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With modifying one of my paladins hour blessings to use
Go Shadow effect. Im pretty much sure that its not what were you looking for, but thats closest that i could manage.
The thing is Voidform as form it self is not a separate model that changes your base shadow transparent looks, its just spell animation (tentacles and stuff) on top of your already passive void model.
I tried swapping out one of my passives with same spell, but i wasnt able to enter command since chat window wouldnt register enter after typing both spell ids, my guess is that some spells are simply not implemented yet or some actions are lua locked.
Im curious, which spell ID have you used when you swaped your PW:S to show as your Voidform? I want to try it on one of my blessings.
I tried both spell IDs for Voidform found under Shadow's specialization, but as in last mention case i simply wont register Enter button.
Voidform
Voidform (Spell)