Hey Feanix,
another approuch to do custom lighting is to fake forward rendering, since deferred rendering can be quite costly for mobiles.
To do this first disable the G-Buffer with the command ‘r.GBuffer 0’. This disables all postprocessing buffers UE generates for deferred rendering.
Now you could create a material parameter collection where you store your sundirection, suncolor etc. You can easily access your sunlight and get the forward vector (that is the sundirection) per level blueprint.
Using this you can create an unlit material and create a dot product between the sundirection and the PixelNormalWS (or VertexNormalWS, per pixel is more precise).
Finally this should be sufficiant as a start and you could convert this shader code into a shader graph http://wiki.unity3d.com/index.php/TeamFortress2Shader.
The interesting part is the function LightingTF2.
Hope it helps.