So it already kinda works if you enable the usdz extension on your site settings!
To make the user experience our TODOs here would be:
Serve .usdz with the proper MIME-type which is model/vnd.usdz+zip.
Allow list the attribute rel="ar" in a elements.
Auto add rel="ar" for .usdz uploads.
2 and 3 can be hacked using a theme-component, 1 needs core support.
That being said:
Looks like it would be easier to package this web component https://modelviewer.dev/ as a Discouse theme-component and it would be a cross-platform thing for everyone.