Deployable Offline WebGL Solution

Uhm, i don't quite get the problem i guess. If you open a website from a local folder the website has access to that folder and it's subfolders as long as you're using relative URL paths.

For example i've made this mandelbrot renderer in Unity and deployed it as WebGL into my public dropbox folder. I can run the index.html using the dropbox link, but i'm also able to just open it locally.

However i'm using FireFox only. If it doesn't work in your browser then it's a pure browser specific problem. The assets are packed into binary files. If the browser is blocking those your only options are either using a different browser or try to change the security settings of your browser. A Unity WebGL build consists only of 5 files in total. If the browser is only blocking those when loaded from a local folder it's the browsers fault.

If you want to file a complaint you probably should direct it to your browser manufacturer who is unable to provide a secure local access for a locally hosted webpage. It seems ridiculous to allow the loading and parsing of binary files from an arbitrary internet location but blocking them when loaded from a local folder. How could that be more dangerous? As long as the access is restricted to that folder / subfolder i don't see any problem. Crossdomain restrictions should apply there as well.

To answer your last question: No, i don't think there's a workaround. The Unity engine core works the way it does and it needs the files the way they are.