> use LD_PRELOAD to wrap fopen/open/openat (I'm pretty sure, ready made solutions for this already exist)

I think I would literally recompile libc to patch fopen/open/openat long before I would even begin to consider writing a kernel module to mess with filesystem paths on a per-process basis.

I feel like if you find yourself seriously considering writing a kernel module then you are either contributing to kernel development, or have embarked on an adventure specifically to learn about kernel internals, or have take a very wrong turn.