Greyman,
I am currently working on this. The "trim ring" around the Hennessey rear facia comes from the factory with it attached - so the trim rings are not made by Hennessey himself.
I talked to Roy at TNT and they used to make them but found the stock rear facia exhaust openings to have varying tolerances on the hole positions and thus stopped making standard exhaust trim rings. TNT will still make one - but needs to have your car in their shop. This is easy for me since I live in Houston.
I am not aware of anyone else making them. Let me know if you find any thing.