Well, I add my fresh experience in installing Win10 on my MacPros (flashed 4.1 -> 5.1 and also 5.1 native), I have not much experience on the Microsoft side but this path worked for me.
1) Choose a dedicated empty internal SATA disk, we are not using BootCamp Assistant and my experience trying various PCIe-SATA hard cards was not so succesful.
2) From OSX Disk Utility format the choosen drive in a single MBR / MS-DOS Fat partition naming it whatever you want (I named "WIN10" for clarity during the next install)
3) Insert the Win10 DVD install and choose from Startup Disk preferences this DVD as boot volume
4) Turn off the MacPro, unplug all the other PCIe/SATA disks except the one you are about to install Win10
5) Turn on the computer and allow the system to boot from the DVD Win10 install (it will takes a long time with that blue window in the middle on the screen.
6) After answered the first questions the installer prompts, you have to choose the disk/partition where install. At this point you should see only one valid disk with one single partition. Select it and Format (do not eliminate, create and format, just format and stop there)
7) Go ahead and let the whole install process happening...
8) Win10 should have drivers updated enough to allow you a interne connection so open your favorite browser and download the well known utility Brigadier
9) Search and Install also the 7zip utility since it is needed by Brigadier for its tasks
10) Launch Brigadier with this argument "brigadier -m iMacPro1,1" and wait until the process finished
11) You will find a drawer named "BootCamp-041-55643" (this is true at present date)
12) Open the Terminal as Administrator and navigate thru the folders until "Bootcamp/Drivers/Apple" section
13) Enter "msiexec /i BootCamp.msi" and go ahead in the install process until the end.
14) Close the Terminal since you don't need it anymore
At this point you could think everything is done but sadly this is not true. While the installer says "I am installing this, that and everything else is needed" you will discover it installs almost nothing but Sofware Update, the BootCamp app (but not the grey square icon in the toolbar) and HFS+ driver (but NOT the APFS and at present I have no idea how to force it to install).
Additionally you will discover that BootCamp utility will not allow you to choose a boot volume and every time you press the "Apply" button it will exit with an error about missing privileges.
Also you have no Bluetooth at all, in Device Manager you can see the yellow triangle near it so let's try to fix as much as possible these troubles.
Missing BLUETOOTH
The driver is luckily enclosed in the $WinPEDriver$/AppleBluetoothBroadcom64 folder, simply open Windows Device Manager, select the broken Bluetooth icon in the list, choose to update the driver manually and navigate to the folder above, select, install and voilà, an happy blue icon in the tray bar and no more errors in Device list.
Missing BOOTCAMP functionality (no APFS)
To enable again the switch between Win and OSX (HFS+ only) you have to tell to windows about the special custom hardware resources a MacPro have and this can be done installing the AppleHAL extension in the Bootcamp/Drivers/Apple drawer. Simply right click on the MacHALdriver icon with the gear wheel, select Install and reboot. After that you will be able to select the Mac partition as bootable volume without errors. Please note that the grey square Icon in the bottom tray bar is still missing, to enable it you have to install the Keyboard extensions, not really sure what but at a certain point it will appear.
Missing APPLE Keyboard Support (wired)
You have for sure seen that "Volume Up/Down/Mute" special keys are not functional and also the eject button does nothing. Well, for the Eject button I have still not found a solution but for the others it will be easy after the install (in this order) of
AppleKeyboard
AppleKeyManager
in the usual driver section with the same procedure seen for AppleHAL. After the installation of KeyManager you will be prompted to reboot and after you should have both the Volume keys functionality and the BootCamp launch icon in the toolbar. If not you may try to install also
AppleNULLDriver
but at this point everything should work fine. As said before the eject button is non functional and I have no idea how to fix it using the official Apple stuff.
For wireless keyboards and mouse you may should try the other specific extensions, for me the above procedure worked for the hardware I have.
The worst part is that APFS support is missing, I have read here and there that BootCamp 6.1 from iMacPro should install it but it did not occour at all.
I have not used the BootCamp installer from 5.1 because it wants to be on Win7 and obviously we are not there so I go straight to the 6.1 (the ImacPro1,1 version)
BEWARE! IF YOU RELAUNCH BOOTCAMP INSTALLER IT WILL UNINSTALL ALL THE DRIVERS ABOVE AND YOU WILL HAVE TO REDO THE WHOLE PROCEDURE AGAIN!
I hope this little guide could be helpful for someone
