We did same what you want to achieve.. in our case also we didn't have access of master db from CD server then we implemented this solution and it is working perfect for us. We have multiple CD's and still we are using the same solution.
In this post we followed below steps -
1. Created a custom database to store comment details.
2. Added a connection string for custom database in connectionString.config file.
3. Stored the comment details in custom database table.
4. Created a Sitecore scheduler to migrate the data from Custom database to the Sitecore content tree.
5. With the help of this approach we don’t need to tweak any of the security settings on the CD server.
Hope this article can help you