would this work as a mono headphone out?
Eurorack level to Balanced Pro Line Level Ouput Shifter in 1U
https://intellijel.com/eurorack-modules/line-out-1u/
would this work as a mono headphone out?
Well, I think you'd need a stereo to mono adapter for that to work. Otherwise the sound would only come out of 1 ear...
ah, of course :) but in terms of volume levels?
I "think" you should be ok. I know I've used an adapter like that in the past, and simply attenuated an output into headphones, so this should be ok level wise. I'm not 100% sure, but there might eventually be a headphone 1u module from intellijel. I know that they have plans for much more in the format.