You need a dummy plug. Yes, there are dummy plugs for 290's. Look for HDMI/displayport to VGA adapters, then make a dummy plug out of the VGA adapter. Getting a digital to analog converter is the only for sure way the thing will work, so I wouldn't try a DP/HMI to DVI adapter, even if it has the analog output pins.
Only need one per rig, luckily, because they are more expensive.
Alright, thanks.
Was thinking about dummy plugs as a possible solution but wasn't sure if it was the only way. Guess there's no other way.