Are there many companies interested in a listing whose quality is questionable?
As everybody knows each other: MBSolution has knowledge in Tryton, they are trusted from my side 
A small hint - on the donation page nobody is listed - this list should be up to date to say thanks to donators.
About the success stories: I think (while writing) we should search for a way to publish kind of ‘good news’ about the project. A nantic event, a use case of GNU Health in the world, a new module on pypi, a finished project and also a single installation like the one from @herrdeh is a good news for Tryton - and all are success stories by its own. Possibly we should think about a simple way to publish news about projects without the burden of formality.
On the subject of service providers: This should never be dependent on a contribution. Someone who implements projects on behalf of a client is by definition a service provider. If one wanted to check it, the Foundation would have to obtain confirmation from the client. And we have the Board of the Foundation, which can decide in case of doubt.