Hmm, actually now that I look closer at the Japan requirements, it doesn't seem to allow replacing the web view systemwide, as I thought, and as Android allows. And neither do the EU requirements. They only allow individual apps to embed an alternative engine on a per-app basis by including the whole engine within the app. And the Japan page includes the caveat "apps from browser engine stewards" which if interpreted zealously (and I expect Apple to) would forbid apps not from Google or Mozilla from embedding Chromium or Gecko.

This is a pretty big limitation considering how much iOS web browsing happens in web views. Having both the EU and Japan as markets may be enough for Google to port Chromium just for Chrome itself, but we will have to wait and see. Actually Chromium development is open so it should be pretty easy to see if Google has a serious porting effort or not.