[racket] Syntax class patterns and nested attributes

Suppose I have these syntax classes:

(define-syntax-class A
  (pattern (a1 a2:B a3)))

(define-syntax-class B
  (pattern (b1 b2 b3)))

I'd like to be able to access nested attributes, like so:

(define foo
  (syntax-parser
    [blah:A #'blah.a2.b1]))

Currently, this doesn't work (and it doesn't even throw an error either).
How could I do this?
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