OK, I gave-up on Diskcopy since both versions split the images...
That said, I made a "fake" 40 GB Dual-Layer BluRay image with Toast, using Mac OS 10, and moved it across to the PC with a USB stick formatted to exFAT (you can't use NTFS or FAT32). SheepShaver saw and mounted the volume perfectly! I assumed that Mac OS 9.2 can't see exFAT volumes, so I didn't bother trying on that platform.
I'm not super-familiar with Toast, so maybe you don't need to bother picking a "target" format that will fit your volume size, not sure, but it works in any case.