The counter-proposal is: don't do it. This comment by the Apple guy sums it up well IMHO:
> Colleagues and I have discussed this and don't see a way to grant write access to the end user's local file system in a way that safeguards the end user's interests.
So they can make it to work for platform-specific apps that they tax 30%, but can't make it to work for interoperable web apps which they can't tax. How convenient.
It's able to support this because the 'file system' is scoped specifically to that origin, and doesn't allow access to any aribtary location on the file sytem, just like iOS.
If Apple made arbitrary file access work for apps [1], surely they could make it work for Safari. But apps pay 30% and websites don't, so it's easy to conclude why they don't want to.
> Colleagues and I have discussed this and don't see a way to grant write access to the end user's local file system in a way that safeguards the end user's interests.