Great work! How does it work on the software side? Does it output just the same as the Line Outs? Can you configure it?
There is no software side 
just connecting!
So the output is the same like on the line outs?
Yes! Simple as that.
I rebuilt it, works great.
My zynthian arrived about 24h ago and I already modded it 
Come on hand it over !!! 
Its basically the same as @Joost_O showed above. It would be nice to have this as official option, it just needs a cable, a jack and a additional hole in the case
It would be even nicer with a volume control.
In my experience the volume is just right and you can adjust with the volume encoder on the UI. Fine for me.
Like, in hardware? Not just a controller for the software volume?
Yep! It is really handy to be able to adjust your headphone level separate to your main output. Each pair of heaphones may have different sensitivity and require different level. You may be feeding the main output to a DAW or PA and want a local monitoring feed. It is good to be able to adjust the monitoring without having to find the right menu / screen.
Maybe something like this from our Chinese friends for a few bucks:
Use the 5VDC from P5 on the hifiberry and connect the “OUT” pins to the stereo jack connector. (I used this one in a DIY organ, works great.)
Seems like this also brings a headphone driver, wont a simple voltage divider do the job? I am going to try this…
Fully agree. it was one of the most used knobs on the audio desks in our edit suites . . .
Sometime it was the only way of working out precisely what ‘sweetening’ the audio lot had added and did it constitute 'Compression ’ . . .
I was able to get the BCM built-in jack to work for headphone output, but it was a fairly quiet output for driving my AKG K240 headphones directly. So instead I used the following external headphone amp connected to the main outs:
Good to know P5 is an option for a dedicated headphone out. A dial (walkman style) volume control would be a nice touch to keep a low profile, but I bet the guts of this box I have could be put into the inside of the Zynthian and be done with it. It has parallel 1/4" and 1/8" headphone outs which is nice, my Audient monitor controller has it as well and it’s a very handy feature indeed.