It is indeed editable, but I don’t feel confident enough on Nifty Gui to propose my edit.
Maybe that’s what others thought before me. I’ll experiment a bit more and get back on it when I think it’s proper time.
Anyway, for the others who will stumble upon this issue:
This is the current wiki Nifty constructor
NiftyJmeDisplay niftyDisplay = new NiftyJmeDisplay(assetManager, inputManager, audioRenderer, guiViewPort);
To use a batched renderer I found the most appropriate constructor is
NiftyJmeDisplay niftyDisplay = NiftyJmeDisplay.newNiftyJmeDisplay(assetManager, inputManager, audioRenderer, guiViewPort);
I don’t want to sound rude, but your solution was already mentioned… in THIS thread^^ I had the same problem about 3 years ago, asked it and it got solved… in this thread… In the last post, when you revived it.
It wasn’t easy though, I had to go through the NiftyJmeDisplay.java file to see what constructor used the batched renderer.
Or scroll up a few cm and see:
Option 2: Enable the batched renderer with this new constructor
Next time just read the whole thing, even long posts may contain something useful. 
Anway, glad to hear it works - All the best for your game!
Yes @destroflyer, but the solution posted in this thread is marked as deprecated in the source code. A few things blurred my mental path to the solution; I’m not even sure (yet) that my solution is the proper one so I won’t pollute the wiki.
I ended up going through the source code, which is fine, but sometimes one needs just the hint for what to look for 
Anyway, thanks again for your help.