There's some stuff I want to do in rendering, which I SceneKit is not letting me do. So I want to build my own render pipeline.Is there a nice way from taking geometry loaded by SceneKit and pushing it into MetalKit for rendering?

I have an idea for a rendering method which I thought would be a good use for SceneKit and Metal. I have figured out how to do DRAW_QUAD and DRAW_SCENE shaders which work.But I am repeatedly hitting problems with a lack of documentation.I want a scene shader technique to render-out a special buffer. This shader needs some custom uniform parameters for each draw call. My technique assocates this uniform with a symbol and SCNTechnique provides a handy callback solution: handleBindingOfSymbol( ) - A block is invoked at each draw call allowing my code to supply custom pararameters ....... Except this is an OpenGL-only call. The block is never called for a Metal implementation. There is no direct Metal equivalent. Is there a way round this? Or should I just leave Metal alone for a year?