Implicit casts to array types

I'm trying to create an implicit cast from an unknown type into a text array
type by creating a simple array of size 1. e.g.

create function textarray(unknown)
returns text[]
as 'select ARRAY[$1::text];'
language sql
immutable;

create cast (unknown as text[]) with function textarray(unknown) as
implicit;

However, when I try to use this, the planner doesn't use the implicit cast.
Instead it still tries to cast 'a' directly to a text[] and complains that
it's not formatted as '{a}' (ERROR: array value must start with "{" or
dimension information)
I added an additional parallel cast from text to text[]:

create function textarray(text)
returns text[]
as 'select ARRAY[$1];'
language sql
immutable;
create cast (text as text[]) with function textarray(text) as implicit;

Now, if I explicitly cast 'a'::text the implicit cast to text[] fires.
However, this doesn't help because I need all the implicit casts to fire
since this is intended to be used by COPY FROM.
I tried adding an implicit cast from unknown to text to try to get
unknown->text->text[], but that didn't work either (same error as first
attempt).
Is there something special about the unknown data type that I'm unaware of?
I don't understand why it worked for text but not for unknown.

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