Failure as well with WinNTSetup.
Actually, i have now ruled out all below items as being troublesome:
-ipxe/sanboot is not the issue,
-the partitioning is not the issue,
-the EFI part is not the issue,
-the BCD is not the issue,
-my win10 1703 is not the issue.
The registry is the issue (in manual mode that is).
When going thru windows setup, this one is smart enough to add a few registry entries :
-a network driver services key (e1express in my case)
-a mounteddevices key (\dosdevices\c: matching my GPT windows part GPT ID in mycase)
and possibly a few other registry modifications which I have missed.
My conclusion at this stage : installing windows 10 on a UEFI "only" hardware thru IPXE/ISCSI works nicely ONLY if you go thru the windows setup for stage 1 (i.e partition, apply files, etc) but it is challenging if you decide to perform stage 1 as manually and you will then get stuck in stage 2 resulting in a windows message "innaccessible device".
Actually, this challenge (manual stage 1 with iscsi) probably applies to all situations and not only to UEFI so this part may have been a disgression from the main topic ("sanboot-windows-on-uefi-system").
Apologies for the disgression then.
A few things I learned thus :
-windows is rather flexible regarding the partitions scheme : you can have 2 to 4 partitions, and you can change partition types and attributes
-you cannot mount a GPT partition with type=system
-you can change attribute and type for a GPT partition back and forth (not sure if this is possible in diskpart thus) - see post here.
-bcdboot will take care of EFI folder and files on your EFI/FAT32 partition
-bcdboot, by indicating the source and destination on the command line, will take care of your BCD applicationdevice/osdevice entries (and although I did not check it out, stuff the disk/part signatures/id in there)
-the registry is needed very early in the boot process, possibly even before we get to winload?
-it seems that no virtual platform supports EFI and ISCSI : sanboot wont register in vmware workstation, sa,boot will register in Vbox but will not be seen anyway once the OS is loaded - probably to ACPI/IBFT not being updated
-while doing some mad googling i stumbled upon UEFI:NTFS from Rufus : may be there is a way have all MS files on one NTFS part (and use a FAT32 part only for UEFI:NTFS).
-it is worth always remembering the boot process (below) when testing/playing with boot matters
-device UEFI firmware->
-disk protective MBR (data only, i.e. first entry in partition table spanning the WHOLE disk)->
-disk GPT table->
-a loader BOOTMGR.EFI or bootmgfw.efi or bootx64.efi either in \boot\efi\ or \EFI\Microsoft\boot\ (or in both places) of the ESP partition (that is normally FAT formatted and NOT mounted to a drive letter)->
-choice in \efi\microsoft\boot\BCD on the ESP partition ->
-\windows\system32\Winload.efi on *any* volume BUT the ESP one->
-Windows install on that volume.
That is probably the end of my journey on this topic