Well, to answer this question if someone comes looking: there's no official aarch64 widevine release, but chromeos just started shipping a 64bit arm version of widevine in the last month. But that version is incompatible with the 16k page size kernel that asahi ships with. There was a report in the asahi irc channel that a manually patched libwidevinecdm.so works.
The fetch-latest-widevine.sh[0] script says 'Architecture not supported'.
[0]: https://github.com/proprietary/chromium-widevine/blob/master...