For everybody: Windows arm64 stuff will be posted in THIS group under "BTW" tags. I have a golden key unlocked Lumia 2520. The plan was, that after W8-1 RT will get the final updates (january - t. y.), then I will make the final custom W8-1 RT ISO, (since constant month-to-month updating has eaten all the space on 30GB C:\ drive and OS is now unusable anyway). (KBs are already downloaded for that, but ISO isn't still made yet, because I had to replace the LCD and the new one arrived and was installed just two weeks ago.) I will have to yet make "the satisfy WU" procedure (to list all needed KBs) and also fully update Office 2013. After the ISO will be made by me, I will continue to try to install W10. There is a group on Telegram and its admin Gustav (focused mainly on phones) gave me the PEP driver from Lumia 1520 (which has the same chipset as Lumia 2520), so the next step is to replace 2520's PEP driver with this one and then boot to W10. There is a chance, that it will not BSOD and work instead. I have to explore the driver 1st, dunno if it is just SYS with user-made INF (then I will make user-signed CAT for it by WDK and SDK) or if it is the complete MS-signed driver with all SYS, CAT and INF. However I believe, it will work quite nicely, then I post the installation procedure here.
Ah, on Lumia 2520, I was only able to boot from other OSes: Windows 8-1 RT Preview b9431 and Windows IoT (two different versions of it). I failed not only with W10, but also with W8-0 RT (both with BSOD). (With W8-0 RT there was clearly the issue, that drivers were not signed for W8-0.) Basically, Lumia has "love at 1st sight" with W8-1 RT b9600 and doesn't want anything else. Maybe in the future some other build of W10 arm32 will be available and we will try again. Anyway, stay in touch and report any news.