bigrocketapps

I have not checked the source code to tell you if the system is the one blocking access or the browser-level API itself. I'm guessing it's the browser. The only reason I mentioned Linux is that's where I tested it and I'm sure there are differences across OSes.

My biggest concern here is the write permission.

If you tried this out in Linux, for example, the system would block you from selecting folders that contain files that are flagged as dangerous or "system files". I'm assuming this was implemented across all OSes.

Currently using this in socket2.me

Not truly supported across all mobile browser currently, but it's certainly better than just one year ago.