Shadermodel 3.0 can be used

[Smaedd]

While the C++-side of the shader only permits the suffix ps20 to be used, it appears nothing is stopping you from renaming a ps30 shader to _ps20b and loading it up like usual.

Seems to work in my testing, I was able to get VPOS into the pixel shader which is exclusive to SM3.0+:

Modified common file:

// common data shared among all screenspace shaders

// up to four textures available for sampling
sampler TexBase : register( s0 ); // $basetexture
sampler Tex1    : register( s1 ); // $texture1
sampler Tex2    : register( s2 ); // $texture2
sampler Tex3    : register( s3 ); // $texture3

// normalized dimensions for each texture above
// (x = 1.0 / width, y = 1.0 / height)
float2 TexBaseSize : register( c4 );
float2 Tex1Size    : register( c5 );
float2 Tex2Size    : register( c6 );
float2 Tex3Size    : register( c7 );

// customizable parameters $c0, $c1, $c2, $c3
const float4 Constants0 : register( c0 );
const float4 Constants1 : register( c1 );
const float4 Constants2 : register( c2 );
const float4 Constants3 : register( c3 );

// eye position in world coordinates
const float4 EyePosition : register( c11 );

// xyz will be 0, 0, 0 (black) if mesh is rendered without fog
const float4 FogColor    : register( c29 );
// range of compressed depth buffer. usually 1.0 / 192
#define DepthRange         FogColor.w 

const float4 HDRParams   : register( c30 );
// exposure scale (bounded by tonemap controller's min/max)
#define TonemapScale       HDRParams.x
// 16 in HDR, 4.59479 in LDR
#define LightmapScale      HDRParams.y
// 16 in HDR, 1 in LDR
#define EnvmapScale        HDRParams.z
// gamma, equivalent to pow(TonemapScale, 1.0 / 2.2)
#define GammaScale         HDRParams.w

// interpolated vertex data from vertex shader, do not change
struct PS_INPUT
{
	// texture coordinates
	float2 uv		    : TEXCOORD0;
	// always (0, 0)
	float2 zeros        : TEXCOORD1;
	// unused
	float2 texcoord2    : TEXCOORD2;
	// vertex color (if mesh has one)
	float4 color		: TEXCOORD3;
	// screenspace position
	float2 screenPos	: VPOS;
};

Shader source:

#include "common.hlsl"

#define SCREEN_SIZE_X ( 1920 )
#define SCREEN_SIZE_Y ( 1080 )

float4 main( PS_INPUT i ) : COLOR
{
	float2 normalisedScreenPos = float2(i.screenPos.x / SCREEN_SIZE_X, i.screenPos.y / SCREEN_SIZE_Y);

	float4 result = float4(normalisedScreenPos, 0.0, 1.0);
	
	return result;
}

[ficool2]

Warning for anyone that wants to attempt to use this: This hack may be vender-specific and I do not recommend using it: the documentation says "you may not use vs_3_0 or ps_3_0 with any other shader versions". The vertex shader is still SM2 and therefore it would violate that rule. I have not been able to get any SM3 pixel shaders running on my end with this so it appears this may be true, I have an AMD card while Smaedd has an Nvidia card.

This needs to be tested with supplying a custom SM3 vertex shader.