something as simple as a gigantic 7-zip file split into volumes might do it for backing up to a list of ISO files. you could, after detecting the creation of a new sequence number, burn the old sequence number and delete it. repeat this until the 7-zip process exits. then finish burning the last sequence number.
there's your program. you can use cdrtools. and it works on linux and windows, and might even work on the mac, because it should be portable. but cdrtools is open source I think. you can implement the burning tools and .iso creation any way you wish.
7-zip has been ported to linux, probably mac, windows, and DOS.
7z.exe has the parameter -v which takes parameters of volume size such as -v4700m (it's in MiB) or -v24g
so the commandline would look like for blu-ray BD-R DL
7z -a -v49g c:\backup.7z
you should include a copy of 7z-920.exe and 7z-920.msi in the first ISO.
the isos should be sized so that the size is reduced on whatever media size they choose to handle fingerprints on edge of media and the size of the 7-zip executables. (700MiB cd-r, 4680MiB DVD-R, 8.5GB DVD-R, 25GiB blu-ray BD-R (SL), 50GiB BD-R DL, 100GiB BD-R XL, 128GiB BD-R XL(128GiB)).
I am thinking that the disc should be filled to 80-90% capacity.
I need to explore the possibilities of restoration from a single sequence file. if this is not possible and file does not span multiple discs,I need to take that up with 7-zip folks.
7-zip project is at https://sourceforge.net/projects/sevenzip/