Ports are all forwarded. When I open the dedicated server launcher, it claims "pak0.pk3 is missing" (it isn't, I'm looking at it in the correct folder listed in the command line interface).
When hosting a server from within the game, it is not joinable despite all relevant ports forwarded.
So this is just totally unusable?

Hey!
Sorry for your experience.
Coop uses completely different engine branch and was developed by different people. That's why it gets much less of my attention simply because RealRTCW still mostly a one man project.
BUT
I checked and RTCWCoopDED.exe is indeed having issues with directory search.
You have two options:
1) Host the server from inside the game. But you mentioned it problematic.
2) Copy all original RTCW .pk3 files manually into "My Documents/RealRTCWCoop/main" (NOT COOPMAIN BUT MAIN). If you do this .bat file will work.
I really want to offer more improvements and QOL to coop. It is mostly a matter of my time and current servers compatibility. But we'll see what future holds. It is a new big task, but still very exciting.
Have a good day!