[racket] is there any way to tell syntax-parse to parse a pattern within a parametization ?

Is there any way to tell syntax-parse to parse a pattern within a parametization ?

For example in something like this:
(define within-y? (make-parameter #f))
(define-syntax-class x
  [pattern _ #:when (within-y?)])
(define-syntax-class y
  [pattern :x])
(define (parse stx)
  (syntax-parse stx
    [:y #t]))

How do I tell it to parse the y syntax-class within a (parameterize ([within-y? #t]) …) form?

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