Thanks @posborne 
I didn’t even know ST supported mDNS, I thought only SSDP worked for now. If there really was mDNS support, device discovery could be handled directly from the hub, there’s not much magic to it. This would also remove the enormous dependencies for mdns in node.js. Chromecast used to respond to SSDP in Cast V1, but that is either not supported at all anymore or will be removed in the future.
The minimal requirements would be the ability to send TCP/UDP packages to a port directly (without the HTTP overhang) and protobuff support. According to the forum the first thing isn’t possible so I didn’t even bother trying to get protobuff to work. Someone who seems to have given up on his DTH already pointed out there might be undocumented protobuff support in ST, but I didn’t check.
I’ll look into adding device discovery from the hub directly in the next couple of days. Running everything of the hub would be great, but I can understand that direct connections to/from the hub may not be a great thing for security.