unity_LightmapMatrix in Cg ?

I know this question is 3 months old, but since it is the only valid page on this topic when googling for it, probably other people will find it too, so I post my findings here:

While I didn't find exact equivalent to unity_LightmapMatrix, I figured out how to compute lightmap coordinates and access lightmap texture from a CG shader.

simpy add the following two uniforms:

 float4 unity_LightmapST;
 sampler2D unity_Lightmap;

then in your vertex shader calculate your lightmap UV coordinates like this:

 float2 lightMapUV= ((v.texcoord1.xy * unity_LightmapST.xy) + unity_LightmapST.zw);

then in your fragment shader, access the lightmap like this:

 fixed3 lightmap = DecodeLightmap(tex2D(unity_Lightmap, i.lightMapUV));

DecodeLightmap comes from UnityCG.cginc

HTH