I found a solution.
LaCie once shipped a LaCie D2 Blu Ray drive.
This drive’s system requirements were Mac OS X 10.3.2 and up.
After downloading and installing the 52KB software (likely a .kext file sitting in /System/Library/Exensions mt Pioneer BDR-209EBK drive which is currently without an enclosure but connected to the Power Mac G5 (OS X 10.5.8) via a SATA to USB (very cheap) adapter shows it’s inserted disk on the desktop.
I installed Toast 10 and clicking the cogwheel on the lower left display the said Pioneer BDR as a viable destination.
A PowerMac G5 can burn BluRay data disks if you install LaCie DiscRecording Support 1.26 for (Mac OS X 10.3.2 through 10.3.x 1.26)[br]
https://www.lacie.com/gb/en/support/dvd/blue-ray/drive/
I hope this helps others solve similar issues and help keep the usefulness of the PowerMac G5 continue at least in the BDR age.
Special thanks to Dan who kept the conversation going.
Kind regards[br]
—Alex