PlayTo is simply Microsoft's implementation of a UPnP control point and server. I don't think there are any APIs for it - if you want to write your own PlayTo implementation, you can do this using UPnP. There is a limited API for UPnP within Windows 7 which can be found here http://msdn.microsoft.com/en-us/library/aa382303(VS.85).aspx
Cheers,
Andrew