I ran into the same issue trying to get triggerhappy to process the VOLUME_UP and VOLUME_DOWN events from my Dell AC511 soundbar. No matter whether I set the user to run the process as in /etc/defaults/triggerhappy, or /etc/init.d/triggerhappy, it would always run as user nobody. The process would run, but because nobody doesn't have privileges, my commands to amixer would not run. The solution I found was to edit /etc/systemd/system/multi-user.target.wants/triggerhappy.service and set the user in there to pi. After doing that and restarting the daemon, the triggerhappy process ran under user pi and everything worked. Edit: Future updates my overwrite changes to this file, I don't know.

Glad it can be helpful for others!

Okay folks, I've written up a script to handle this exactly the way I described before. It runs from the autostart.sh and watches /dev/shm/screen_manager.cfg for updates. That way any other system process can create/modify that file and this script will respond by either starting the rom, or starting the dashboard. Cheers! https://github.com/coderkevin/mini-nes/tree/master/screen

Thanks for the info. Can't wait for my Pi 3 to arrive!