It's not as much "non-exit" as Tor middle relays. Because it just connects to the VPN2 server using OpenVPN over standard TCP/IP. Instead of some proprietary protocol. But at least it's locked down with pf rules, so that it can only connect to the VPN2 server.
The diagram shows a nested chain with just two VPNs. But you can add more layers. As I recall, as many as six or so. Latency goes up, and MTU goes down. But throughput doesn't crash as much as you might think. I don't know why. But maybe it's caching.
So basically, you have a NAT chain locally in VirtualBox or whatever. And each NAT router includes a remote VPN server.
In order to share it, you'd need to open a port for incoming OpenVPN connections. Either locally, or forwarded to one or more VPN servers. And then you could route traffic through another VPN server in the chain.
What about sharing it as an internal non-exit VPN in a nested chain?