Okay, so it's not the best work around but I actually got this to work finally!
First do the obvious -- make sure Windows 8.1 is fully up to date and Bootcamp 6 is installed. The only way I could update to 6 was to run the Bootcamp assistant on the Mac side and create a driver USB flash drive which I then used to update Bootcamp on Windows. The update notification in the Windows notification center was taking me nowhere.
Now -- how to upgrade to 10. Well, I run VMWare on a laptop. I never used it to boot a Bootcamp partition but in the back of my mind I was pretty sure that was a feature so I installed a copy on the iMac and sure enough. It was pretty easy to boot the Bootcamp partition with VMWare Fusion. It took a long time for Windows to sort things out and yes, Fusion wanted to install its driver set and so on but when things finally settled down I attached my Windows 10 install media on a flash drive and started it...and it actually worked! When it was all installed and I verified I could login to my account I shut the VM down and then rebooted into Windows 10 direct. If needed to reset the drivers back to the native ones but everything was not only working but I did NOT have to reinstall Bootcamp 6 -- it was still up and running from 8.1!
Again, not the best work around but I figured if I tried upgrading while in a VM then it wouldn't see the fusion drive and the theory seemed to prove out.