Suggest setting up a notice for new users when going into the main web hosting forum. Perhaps for users with 0 to 20 posts. Something to the effect:
"If your posting a review of or comments about your Web Hosting Company, please include the domain name that you host with them in your post. If for some reason you need to keep this information private, once you have posted your message, please click report post and supply your web site to the WHT staff in a confidential report. Please don't take offense, we ask this of everyone to maintain a fair environment and prevent fictitious reviews."
Seriously though, a required field below the title bar asking for the URL would do the trick (could keep the field private and allow them to mention the URL in the message if they want it public).
e.g. "Please enter the URL of the web site that you have with this provider, this field is kept private, it's only used to verify that you are a client of this provider, if your want URL to be public, you may mention your URL in your post."
It would only be necessary when a new thread is started and only in the main forum.
I know it's some custom programming, but it doesn't appear that it would be all that difficult of a mod.