WinSpd is not a standalone application, but rather a set of libraries and components that allows others to create applications that can do what you want (serve a disk over a network).

The included sample, called rawdisk, memory maps a file and uses it as a disk image file. This could presumably be done over SMB or NFS, although I have not tested the performance.

Yeah. I thought of putting my Steam game library on my NAS so I can use the same library across multiple computers, but Steam doesn't like having its game library set on a network share.