I was able to get Boot Camp back on my Mojave updated 2013 iMac w/3TB Fusion drive. It's time-consuming and ugly, but it worked.
I believe it's now using EFI booting instead of legacy booting since the Bootcamp partition is past the 2TB boundary on the Hard Drive.
- Mounted the Bootcamp.dmg file and created an image file of the mounted disk image using Winclone 7.
- Cloned/backed-up my Fusion drive using Superduper.
- Booted from backup.
- Rebuild Fusion Drive as HFS+. (there's an Apple Tech Document on how to do this... not in my notes)
- Clone backup to the HFS+ formatted Fusion drive.
- Install a fresh Windows 10 using Boot Camp Assistant.
- Restore Winclone image file back to the new Bootcamp partition.
Good luck!
FYI: This is all with a small (~240G) Bootcamp partition
Looking ahead does anyone have an idea how to pull this off with the 1 TB fusion drive from 2013 iMac? Also, is this setup likely going to break with every Mojave incremental update?