guitar, I'm not sure what you mean. Is your shader something like this?

VS_OUTPUT vs_main(float4 Pos: POSITION)
{
VS_OUTPUT Out;
Out.Pos = float4(Pos.xy,0,1);
Out.texCoord.x = 0.5*(1+Pos.x-viewport_inv_width);
Out.texcoord.y = 0.5*(1-Pos.y-viewport_inv_width);
return Out;
}

I'm not great with shaders, I'm afraid. Could you tell me if you can do each of the following?

Can level entities go in front of and behind your shader output? (Does this render plane have a z-depth in the level? If it does, how do you give it one?)
Can you stack multiple such render planes at different depths in the level with transparency?
Can you animate the texture, either from avi or from a single image like sprite animation?

I like that a solution that uses sprite or sky entities would come with the feature set that these types have. preload_mode and easy script control over position and animation...

(Come to think of it, does preload_mode apply to ent_createlayer?)