http://www.tinyclr.com/forum/topic?id=8528&page=1
http://www.tinyclr.com/forum/topic?id=8678&page=1
What you need is all achievable; seems to me you’re over thinking this.
"You need the serial shield to communicate with the Domino from Visual Studio or MFDeploy when the mode is Serial " is the full quote. If you don’t need to do debugging over the serial line, you don’t need another serial port (although seriously, you can never have too many serial ports !!! 
)
So again my question to you is WHY. You said “sensors, motor driver” which doesn’t mean anything, sorry. It’s more a question of what communication you need to do over the link that’s important.
Again, let me say this clearly. CDC is supported, and you haven’t said you need anything other than virtual serial, and there’s sample code in http://wiki.tinyclr.com/index.php?title=USB_Client#CDC_-_Virtual_Serial that will show you exactly what you need.
The driver is most likely your problem here. But before we leap to that conclusion, can you show us your code (or go back to the example I linked to).