I found it. It blows my mind. Let me just cry out here.

    (define (partition->dos-type partition)
      ;; Return the MBR partition type corresponding to the given PARTITION.
      ;; See: https://en.wikipedia.org/wiki/Partition_type.
      (let ((flags (partition-flags partition)))
        (cond
         ((member 'esp flags) "0xEF")
         (else "0x83"))))

    (define (partition->gpt-type partition)
      ;; Return the genimage GPT partition type code corresponding to PARTITION.
      ;; See https://github.com/pengutronix/genimage/blob/master/README.rst
      (let ((flags (partition-flags partition)))
        (cond
          ((member 'esp flags) "U")
          (else "L"))))