For anybody that is interested, there is a new addon for HBO Go EU.
It is still in beta but very promising. The latest version beta7 works very well on my X92_S912 box. It uses Inputstream Adaptive and does hardware decoding in 1080p (with subtitles) very well.
On my box I already had Netflix addon installed and working in 720p (software decoding), so there was already needed “libwidevinecdm.so” in the right place.