raspberripi.mongonta.com posted again, this time describing how to boot from USB SSD or HDD. ラズパイでWindows10 on ARMを動かす（上級者向けUSBブート編） — translates as "Running Windows 10 on ARM at Raspai (Advanced USB Boot Edition)". Very careful, step-by-step procedure for doing the installation.
INpact-hardware posted about the project. Comment installer Windows 10 on ARM sur un Raspberry Pi 3 (microSD et USB) — translates as "How to install Windows 10 on ARM on a Raspberry Pi 3 (microSD and USB)". Most of the article is behind a paywall, but will be freely available in 30 days.
Update 2019-02-24: fix typo for INpact hardware, add note about their access policy.