if you already have the uv-map for the light map then it isn't much harder than doing it for terrains (med supports automatic atlas mapping now so this could be used for the uv-map).

i think basically you just need a triangle rasterization algorithm and you have to know how to use barycentric coordinates so you can get the 3d-coordinates of the rasterized pixels. both things aren't too difficult and you can find information about it with google.

you have to use lite-c because with it you have access to the mesh data. i posted an example in the lite-c forum a while ago.

an ingame light mapper would be nice!