Frees up cheap disk space by moving the library contents to expensive RAM? That doesn't seem like a good thing. And it also doesn't seem possible — you want system libraries to survive reboot. Presumably these files exist somewhere on disk, just hidden.
Not hidden, just inside the ROSP partition at /System/Library/dyld.
The dylib cache is stored on disk. It's saving disk space by only having that copy of the libraries instead of two copies.