カスタムポートを通じて脱出。

custom-port-escape.scm

#!r6rs
(import (rnrs))

(define (test)
  (call/cc
    (lambda(cc)
      (let ((in (make-custom-binary-input-port "consumer" cc #f #f #f)))
        (get-u8 in)))))

(define (print . x) (for-each (lambda(x)(write x)(newline)) x))

(call-with-values test print)