Sure but this currently means a virtual authenticator that puts unwrapped passkeys in the same memory as other applications, leaving only the OS and no other physical measures to protect a direct-memory read. That might be acceptable for your threat model, but no other currently adopted WebAuthn authenticators work this way other than password managers that don't want to be left out.

Hopefully in the future OSs will have better APIs to allow third-party tools like 1Password to leverage hardware components like TPMs and Secure Enclaves to build a sync fabric that's not tied to the hardware vendor, but this does not currently exist and is not trivial to implement without significant consideration about phishing.

I think YubiCo and password managers have a tremendous opportunity here to partner up to build sync fabrics bypassing OS vendors that might not be as incentivized to provide these APIs now, but I don't believe it's moving, currently.

Additionally, when you say you wouldn't trust Google or Apple to be the _only_ place your passkeys are stored, it's likely that even if we can have third-party sync fabrics, that these will never be interoperable. I don't believe you'll be able to "export" a Passkey from your iCloud ecosystem and import it into the 1Password ecosystem as you can do today with passwords. Doing so would break assumptions about the strength of the authenticator as far as WebAuthn is concerned, and would weaken Apple's security posture as well. Instead you'd probably have to maintain _both_ sync fabrics independently with every service you sign up for.