App Broker as front end tool - reference customers
If you're talking about App Broker on-prem, we have many customers using the App Broker UI as their user-facing catalog. If you're talking about App Broker Cloud, we only recently introduced our own App Broker UI for cloud and are in the process of implementing that with our first customers now.
As for references, perhaps some customers on this forum would be willing to talk to you about their experiences. Otherwise, please reach out to your Flexera account manager (Matt Schaefer is yours), and he/she will be happy to connect you.