IIS has little or nothing to do with SPDY. IIS is just an application server that responds to HTTP requests handed off by the http.sys kernel mode driver. All HTTP requests in Windows are handled by this driver.
This is the level at which SPDY would be need to be implemented.
If you were to implement SPDY you’d need to have this as a shim driver between the TCP stack and http.sys, or maybe even write your own http.sys driver.
Alternatively you could write your own SPDY/HTTP stack but if you wanted to use this with IIS then you’re in for a lot of work.