Embedded USB Host connection to Windows Mobile

Question

Hello all -

I am charged with designing an interface between an embedded USB Host device (of our own design) and Windows Mobile 6+.  This interface will need to allow us to stream data between the accessory and a running application, as well as do some file operations (read/write/copy).  After stumbling around through documentation and forums, I believe I know just enough to be dangerous.  It looks like that the only usable interface that is exposed from a mobile device once enumerated is an RNDIS interface.  This seems to be the interface that ActiveSync uses.

I have looked through the RNDIS specification and believe we can implement the control/data channel messaging in our host device, but my question is where do we go from there?  What are the higher level protocols that would allow us to transfer data between the host and an application?
Is the interface in the mobile application something like a TCP listener?
What do I listen for?
For file operations, is there information to help me map the RAPI functions into RNDIS operations over the USB?

Basically, what I'm looking for is further information/documentation that describes the interfaces that are normally hidden inside Windows drivers (since that's what we will have to implement on the host).  Microsoft documentation seems to focus heavily on the PC principle, which assumes the developer can make use of their drivers.

Thanks very much in advance for any further direction!
Dave