I have a SSM-D2 dongle that I need to connect to my HAOS which runs on a Hyper-V server. Hyper-V does not support USB passthrough so that is out of the question.
I connected it to a Raspberry Pi running Ubuntu 22.04LTS and shared it over tcp using ser2net. I can connect to it just fine using telnet from my PC, so that side is sorted.
Very simple question now: How can I mount it in HAOS to allow ramses_rf integration to use this device remotely?
You’re kinda out of luck. HAOS doesn’t support installing additional packages like ser2net or socat for device redirection. Your best option might be using a Supervised installation, which will be more effort. That choice will allow you to install whatever packages you want, but you have to maintain the OS at that point.
Some integrations support TCP connection strings in the device config, if that’s the case HAOS will work, but if it expects a serial device you’re stuck.
Since you’ve already picked “the road less taken” with Hyper-V, going with Supervised installation isn’t that much effort. There are plenty of threads on the board on how to accomplish it. I ran it as version 2 after Raspberry Pi (v1) and before Proxmox/HAOS (v3). There are also threads here about using an IP-based USB redirector on Hyper-V, if you feel like diverting into the road even less taken. 
Well I need to look into it then. I’ve been running my home server with multiple VMs on it on Hyper-V for like 4 or 5 years now. I could switch to ESXi but I really don’t want anything to do with Broadcom. There is also Proxmox but I’ve never got around to play with it.
I will google Hyper-V redirectors, maybe that will work. Thanks.