> Or is it not intended to do so?
No, it's not.
The online demo server is indeed an advanced fork of plantuml-server.
There is nothing very complicated or secret about this advanced fork.
Our main problem is that we do not have the time to maintain this code publicly.
However, we would be happy to provide our code if someone wants to create a branch on plantuml-server to add new features.
(i.e. to add existing features on the online demo server).