So after doing some more research I came across the 'Color op' page in the Wiki and there I found the following blend option:

Result = Texture1 + Texture2 - 0.5
ColorOp[n] = AddSigned;

Looking at the example and assuming that texture 1 should be mirrord(in the wiki, right?) I think this is what I need. If I just go for the static or even the dynamic reflection, I could use this blendmode with the standard environment cube reflection shader from the templates.

Like this:

bmap bcube=<cube+6.tga>;

function mtl_envmap_view()
{
mat_set(mtl.matrix,matViewInv);
mtl.matrix41=0;
mtl.matrix42=0;
mtl.matrix43=0;
}

function mtl_envmap_init()
{
//...maybe you should copy the mat_model properties here...
bmap_to_cubemap(mtl.skin1);
mtl.event=mtl_envmap_view;
mtl.enable_view=on;
}

material mtl_envmap
{
skin1=bcube;
event=mtl_envmap_init;

effect=
"
texture entSkin1;
texture mtlSkin1;
matrix matMtl;
technique envmap
{
pass p0
{
texture[0]=<entSkin1>;
texture[1]=<mtlSkin1>;

zWriteEnable=true;
alphaBlendEnable=false;

colorArg1[0]=Texture;
colorOp[0]=AddSigned;///////////////just change this from modulate 2x tot AddSigned
colorArg2[0]=Diffuse;

colorArg1[1]=Texture;
colorOp[1]=blendCurrentAlpha;
addressU[1]=Clamp;
addressV[1]=Clamp;
texCoordIndex[1]=cameraSpaceReflectionVector;
textureTransformFlags[1]=Count3;
textureTransform[1]=<matMtl>; // transform camera space back to world space
}
}
";
}

The cube map would need some tweaking and colour adjustment in photoshop, but it should give a pretty good result.

If I could connect this to a normal mapping shader then it would be perfect.