Silly, isn’t it? Mine is fitted using one of those washers shimano give with their rotors that have two holes like this-
http://www.sjscycles.co.uk/shimano-deore-xt-br-m755-rotor-bolts-and-plate-818-9803-prod30047/
I paint it black, put one of the rotor bolts through one end which I then screw into the internal routing hole for the command post. This self taps but I don’t do it up mad tight, just so the bolt head is flush. Then I attach the hose to the other end of the washer with a zip tie, thread it between the shock mount then through the guide hole on the seat clamp.
The washer I point to the left as it fits flush with the undsersode of the top tube. Its then quite hard to notice, I have only had one person comment on it and it is neater than many zip ties.
You may be able to find a neater attachment than the washer using a bolt threading into the same hole but I swapped the reverb from another bike and the hose was too short so mine had to go from the bar to the left side of the headtube.