Custom shader

Is there a relatively easy/quick way to change the base pass shader for particular mesh?
I would like to use my own main function for VS and PS bypassing all of the the built-in templates.

If it’s not feasible, is there any way to simplify the editor-made shaders?
Unless I understand it wrong, a simple, color only, unlit material shows 43 PS instruction + 31 VS instructions?

In UDK there was a way to code custom node, maybe there is the same on UE4, but if so, you have toi avoid such things because this is not portable on different platform.

Windows -> Directx -> HLSL Shader language
Mac, Android, IOS -> GLSL Shader language

You can do custom nodes in UE4 too, but AFAIK it still goes through the BasePass shader.
It’s windows only for now, so not concerned with cross-platform very much.