It’s all a bitvery rough around the edges, but the compiler is working enough to build opam, just need to update the various support packages and it should then be possible to create switches with it:
$ file opam.exe
opam.exe: PE32+ executable (console) Aarch64, for MS Windows, 8 sections
$ opam config report
# opam config report
# opam-version 2.4.0~alpha1~dev (1eb3e278fb66fbd66a4c5c15af59d9d17a2d7104)
# self-upgrade no
# system arch=arm64 os=win32 os-distribution=cygwin os-version=10.0.26100
# solver builtin-0install
# install-criteria -changed,-count[avoid-version,solution]
# upgrade-criteria -count[avoid-version,solution]
# jobs 11
# repositories 1 (http) (default repo at bc512a2d7545dee0dc96628bd52cddc17739991c)
# current-switch none set