Forward rendering and pixel depth offset


I have a plane facing my camera for which I use the “pixel depth offset” node in order to set the depth. With the deferred renderer I have lighting and shadows on my plane but when I switch to forward, my plane is totally shadowed when I look at a shadow and I don’t have any other light.
I tried to edit the “BasPassPixelShader.usf” code which seems to use the depth of the plane and not the depth + offset. But the only thing I got is a grid of shadows, and I still can’t see the lights.

Is this a limitation of the forward renderer or is there a fix ?