So a small update here…
I’ve managed to get Plexamp running… but again I run into a blocker: the whole authorization flow (getting a claim code) is blocking a fully headless installation.
Yes, you read it correctly: the product they claim is headless is not so headless during installation.
So this is again a no-go for me. And no, I’m not going to contact Plex about this. The last time I had contact with them they made it pretty clear that they are totally not interested in helping out parties that want to integrate their product.
Oh… if I sound a little bit frustrated… then that’s correct 
ps: I’m not the only one. The Hifiberry guys also made it clear that this is a no-go for them.