(sorry for necro) I have a solution which should work on any GPU. dgVoodoo2 (used by Ephinea's higher DirectX modes) can force the game to accept more resolutions than GPU drivers report. You can add up to 16 in comma-separated WxH format -- example 
dgVoodoo.conf below:
	
	
	
		Code:
	
	
		[DirectXExt]
ExtraEnumeratedResolutions = 5760x1080,3840x2160
	 
 The Ephinea Launcher currently generates a fresh 
dgVoodoo.conf on launch, which prevents this tweak.
As workaround, make sure other settings are exactly as you want them, then add the resolution, save, right click the .conf file > Properties > enable Read Only.
If that does not work, set Ephinea to DX8 mode and manually install dgVoodoo2 (though this will disable the awesome HUDless Bloom effects).
@Sodaboy if you want a permanent fix :
When a Custom Resolution is written in Launcher, use its values for a matching 
ExtraEnumeratedResolutions = {W}x{H} in 
dgVoodoo.conf / [DirectXExt]