Setting resolution on RX460 when no display is connected
I've got an RX460 plugged into a tower that usually runs headless. After a reboot, it comes up at 640x480, and is listed as a "Mobile PC Display". I am unable to change the resolution at all. The only changeable settings available through the Windows 7 Display Options are refresh rate (59hz or 60hz) and color depth (16 bit or 32 bit). The display section in the Radeon settings app says that no display is connected. Correct.
I currently have an antique Techwood LCD plugged in on HDMI, and if I switch this screen on for a moment, everything works correctly - it's autodetected as a "19_LCD_TV", and I can then change the resolution to match whatever I'm using to remote in. I can switch the screen off again, and it continues to work correctly until the next reboot.
So, my question is this: how can I make this setup work consistently without needing to leave a screen plugged in just to make my video card work? There is no VGA-out on the card, so a dummy VGA plug won't work.