That does put a different complexion on the matter, indeed. But to continue with the HDI-30/something cable idea, you can reduce expense considerably, and increase versatility, if you get hold of an HDI-30M/CN-50F switchable adapter, such as this one. DOCK and SCSI are the switch positions, which connect in/switch out pin 30, the pin that signals SCSI Disk Mode to the ROM of the PB. In 29-pin mode (SCSI) the adapter is for use with peripherals, such as your mooted external CD-ROM drive. You will still need two cables, a CN-50M/CN-50M for attaching peripherals to the adapter, and a CN-50M/DB-25M for SCSI Disk Mode (DOCK) to another Mac, but these are standard cables likely to cost less than those with HDI-30M at one end. Browse the linked site for prices of other combinations to do the job.
Another wrinkle (you did get your PB voluntarily, now ...) is that you may need a passthrough CN-50M/CN-50F terminator also, to fit between cable and switched adapter, just against the possibility that the lack of termination for older PBs' external SCSI chain plays up, which is often. If you do need termination at the PB end, you must use a switched adapter instead of a cable with HDI-30, because there are no (known to me) HDI-30 terminators.
In the hope that your eyes are unglazed by all of this, jolly good fortune to you.
de