Welcome to the CHICKEN Scheme pasting service

(with-eval-after-load 'scheme
  (defun my-scheme-region-extend-function ()
    (when (not (get-text-property (point) 'font-lock-multiline))
      (let ((new-beg
             (save-excursion
               (when (and (re-search-backward "#>\\|<#" nil t)
                          (equal (match-string 0) "#>"))
                 (point))))
            (new-end
             (save-excursion
               (when (and (re-search-forward "#>\\|<#" nil t)
                          (equal (match-string 0) "<#"))
                 (point)))))
        (when new-beg
          (setq font-lock-beg new-beg))
        (when new-end
          (setq font-lock-end new-end))
        (when (and new-beg new-end)
          (put-text-property beg end 'font-lock-multiline t))
        (or new-beg new-end))))

  (defun my-scheme-syntax-propertize-foreign (beg end)
    (when (search-forward "<#" nil t)
      (put-text-property (1- (point)) (point)
                         'syntax-table (string-to-syntax "> cn"))))

  (defun scheme-syntax-propertize (beg end)
    (goto-char beg)
    (scheme-syntax-propertize-sexp-comment (point) end)
    (funcall
     (syntax-propertize-rules
      ("\\(#\\);" (1 (prog1 "< cn"
                       (scheme-syntax-propertize-sexp-comment (point) end))))
      ("\\(#\\)>" (1 (prog1 "< cn"
                       (my-scheme-syntax-propertize-foreign (point) end)))))
     (point) end)))

(defun my-scheme-mode-setup ()
  (setq font-lock-extend-region-functions
        (cons 'my-scheme-region-extend-function
              font-lock-extend-region-functions)))

(add-hook 'scheme-mode-hook 'my-scheme-mode-setup)

(with-eval-after-load 'scheme
  (defun my-scheme-region-extend-function ()
    (when (not (get-text-property (point) 'font-lock-multiline))
      (let* ((heredoc nil)
             (new-beg
              (save-excursion
                (when (and (re-search-backward "#>\\|<#\\|#<[<#]\\(.*\\)$" nil t)
                           (not (get-text-property (point) 'font-lock-multiline)))
                  (let ((match (match-string 0))
                        (tag (match-string 1)))
                    (cond
                     ((equal match "#>") (point))
                     ((string-match-p "^#<[<#]" match) (setq heredoc tag) (point)))))))
             (new-end
              (save-excursion
                (if heredoc
                    (when (and (re-search-forward (concat "^" (regexp-quote heredoc) "$") nil t)
                               (not (get-text-property (point) 'font-lock-multiline)))
                      (point))
                  (when (and (re-search-forward "#>\\|<#" nil t)
                             (not (get-text-property (point) 'font-lock-multiline))
                             (equal (match-string 0) "<#"))
                    (point))))))
        (when (and new-beg new-end)
          (setq font-lock-beg new-beg)
          (setq font-lock-end new-end)
          (with-silent-modifications
            (put-text-property new-beg new-end 'font-lock-multiline t))
          (cons new-beg new-end)))))

  (defun my-scheme-syntax-propertize-foreign (_ end)
    (save-match-data
      (when (search-forward "<#" end t)
        (with-silent-modifications
          (put-text-property (1- (point)) (point)
                             'syntax-table (string-to-syntax "> cn"))))))

  (defun my-scheme-syntax-propertize-heredoc (_ end)
    (save-match-data
      (let ((tag (match-string 2)))
        (when (and tag (re-search-forward (concat "^" (regexp-quote tag) "$") nil t))
          (with-silent-modifications
            (put-text-property (1- (point)) (point)
                               'syntax-table (string-to-syntax "> cn")))))))

  (defun scheme-syntax-propertize (beg end)
    (goto-char beg)
    (scheme-syntax-propertize-sexp-comment (point) end)
    (funcall
     (syntax-propertize-rules
      ("\\(#\\);"
       (1 (prog1 "< cn" (scheme-syntax-propertize-sexp-comment (point) end))))
      ("\\(#\\)>"
       (1 (prog1 "< cn" (my-scheme-syntax-propertize-foreign (point) end))))
      ("\\(#\\)<[<#]\\(.*\\)$"
       (1 (prog1 "< cn" (my-scheme-syntax-propertize-heredoc (point) end)))))
     (point) end)))

(defun my-scheme-mode-setup ()
  (setq font-lock-extend-region-functions
        (cons 'my-scheme-region-extend-function
              font-lock-extend-region-functions)))

(add-hook 'scheme-mode-hook 'my-scheme-mode-setup)