Is it possible to access the memory the GPU is allocated on the CPU? For instance if I'm running a 3d application and I wish to edit a lookup table which is in a texture, is it possible to access the memory without having to go through the graphics api?
No, and that's more or less how it should be. On boot, the loader reads the
config.txt file and assigns the split for the CPU/GPU. All of this happens within the loading process before the actual Linux kernel starts. Because of this, the kernel itself doesn't have access to the GPU memory space.
In short, it's probably technically possible, but it would require some kind of exploit.