My recomendation is to horseshoe this to a standard user+password implementation, becuase people that know more about security than you and me have thought about it.
Perhaps your server can just ignore the username, so everyone is forced to be anonymous.
The file you are proposing is a password. It's just a strong password generated by the server intead of a hand made silly password.
Usualy there is an implicit user->salt table in the server (that is the first part of the stored hash). So you will have unsalted passwords, that is probably not a problem if they are really strong and are not reused.
If you use a standard ignored_username+strong_password then the browser or a password administrator can keep them and makes the user life easier. (I'm not sure about the supporr of server generated passwords in browsers and administrators.)
Note: bcrypt or scrypt are hashes designed for passwords. They are relatively slow to prevent brute force atacks. The slownes can be regulated and they include salt. https://en.wikipedia.org/wiki/Bcrypt https://en.wikipedia.org/wiki/Scrypt
I agree. Good luck with your project. Remember to post it here when it's working or when you have an interesting blog post about an update in the advance of the implementation.
Anyway, as my father used to say, write everithing as if it will be published tomorrow in the front page of a major newspaper of your country.
One of my big concerns is moderation and legal, as I know cybercriminals/scammers will see my platform as a viable tool for their operations. I'm thinking blocking known malicious IP's, maybe some kind of regex system. Ill still new to this so if you have an suggestions, I am desperate to hear them.