It would be interesting if the networking model for the end targets could also be inverted, so that an agent (or something) on the end target could make an outbound connection to establish a reverse tunnel to the proxy that user connections could then be sent over.
The use case I'm thinking of is for IoT or robotics, where you have devices you want to manage being deployed into remote networks that you don't have much control over. It's really helpful in this situation if devices make outbound connections only, so that network operators don't have to configure their firewalls to port forward or set up a VPN.
Edit: clearer language