Cannot load index.html on cdvfile:// protocol

In the most recent versions of iOS, the cross-domain restrictions on file: have been tightened up. It is impossible to do things like play a video which qualifies as same-origin (and therefore can e.g. be uploaded as a WebGL texture). Accessing the video via the file: protocol counts it as cross-origin; even if you use cordova-plugin-file to read it in to a blob and then use createObjectURL(), it still counts as cross-origin and ends up blocking some features.

If we could load index.html on the cdvfile:// protocol, then we should be able to load all subresources as relative to cdvfile:// which should then count them as same-origin. This would also simplify development since things like fetch() calls should work (they are blocked on the file: protocol).

However I cannot find any way to navigate to a cdvfile:// URL on iOS. Even with 

<allow-navigation href="cdvfile://*" />

in config.xml, trying to navigate to cdvfile:// throws an "unsupported URL" error. This ought to be supported.