Is ther any way to find out the pointer in the GPU memory of the final renderer (BGE) so it can be retrieved by other apps ? (via spout / syphon / vvvv etc)
The GPU memory handling is done by the graphic card driver and OpenGL. Basically the user don’t have access directly to the GPU memory, that is the reason why no pointer to the GPU memory is existing.
But there some OpenGL commands to get the rendered pixels or to render the image into a texture.
You can use glReadPixels to read out frame buffer into the PC memory. But this is very slow.
https://www.opengl.org/sdk/docs/man/html/glReadPixels.xhtml
https://www.blender.org/api/blender_python_api_2_78a_release/bgl.html?highlight=glreadpixels
Or you can use bge.texture.ImageRender. Which is faster (it also use glReadPixels but it is written in C++) and easier to use.
https://www.blender.org/api/blender_python_api_2_78a_release/bge.texture.html?highlight=imagerender#bge.texture.ImageRender