Found the time to test it. Took a while due to christmas and some adaptations needed.
I got it to work. Steps to reproduce
1. Start the RendezvousProxy on any PC in the local network
2. Go to settings and add a new protocol: "_rsp._tcp." (exactly the part between the "")
3. Add the IP and port of your remote mediaserver under that service
4. If you have a firewall or NAT router on the remote side then a port forward has to be properly set there.
5. reboot the roku
Then it will quite seamlessly play also the remote library, proving that it's really only the discovery thats missing in the rokus.
There are 2 gotchas to observe that Roku hopefully fixes at some point:
1. Password protected shares accepted only if you dont connect to em while they have no password. If you do and then enable the password you need to totally unpower or factory reset your roku as it saves the empty password and then tries to use it, not recognizing that now a password is set. That is a bug and I submitted it as such. Workaround: Unpower/factory-reset then reconnect.
2. No way to enable the roku to discover the server without the RendezvousProxy. Please add a way. Could be as simple as allowing a rsp://mediaserver.dns:3689
in the presets. Right now you need to run some sort of computer to enable the discovery. And unfortunately the roku looses contact once the Proxy is shut down.
Those who sacrifice freedom over safety deserve neither.