I have 2 x r9 270x cards in a rig with Ubuntu 14.04 (the old fglrx drivers give much higher performance for those older cards and 14.04 is the last version with fglrx). I can get it running headless but it isn't reporting temperatures - zecminer reports ADL initialization error. To get it running headless I have to kill the X server first ("service lightdm stop"). After that it mines fine, but i want to see the temps. I'm also not clear if it's obeying my config -ttli temp limits because of this.
My rig is running Debian 8 as it's old enough to have Xorg 1.16 and Fglrx in its repo while new enough to use Systemd with support to 2020. An X session needs to be running for libatiadlxx to initialize to monitor temps, fans and reclock.