This message was posted on behalf of Intel Corporation
I have checked your motherboard specifications and I noticed that you are currently running an Intel® server chipset. Based on the platform, I would recommend to contact your motherboard maker, in this case Asus support. https://www.asus.com/Motherboards/P10S-WS/HelpDesk/
Try posting this inquiry at: https://vip.asus.com/forum/
Thank you for your response. Sadly the ASUS "Workstation" support hasn't been able to help me with various issues so far. Their "VIP Forums" are a joke; to create a topic you have to select the specific motherboard model from a drop-down list and half of their range is not on there (i.e. the P10S WS).
Regarding this topic the first response was that they maybe try to try to install Windows 10 but they surely expected everything to work fine.
I could figure this one out myself over the weekend:
1) Create a Windows 10 Pro ISO file with Microsoft's Media Creation Tool.
2) Use "NTLite" (seemed to be the easiest graphical way) to open that ISO file and add the RSTe driver (Version 220.127.116.118, dated May 21st, 2015) to it.
3) Create a new ISO and flash it to a thumb drive
Now, the Windows 10 Setup can handle a RSTe volume as a destination for Windows. The various error messages (check BIOS etc.) were only an artefact of Windows not handling the SATA RAID controller properly with Microsoft's generic Intel SATA driver.
But one questions remains: Why does Windows reject the manual adding of the same 18.104.22.1688 driver during a Vanilla Windows 10 Setup (error message "No signed drivers found")? The driver is WHQL-certified and to my knowledge there is no newer one publicly available anywhere.
Any news on that? The past twelve months have seen a couple of "normal" RST updates but nothing regarding RSTe.
To whole ordeal showed me that ASUS not once during their testing phases bothered to install Windows 10 on that workstation motherboard model while its SATA controller was in RAID mode - and yes, Windows 10 is officially supported.