Hello and sorry for no answer for long time, I have just found your message..
You do not need to do any desktop programming - only Teensy side needed.
Just take the sample sketch from NMEA2000 library and change it so that it will forward all incoming n2k messages to USB serial port.. Then at the OpenCPN go to settings, choose add connection and choose USB serial port. and you will have all the pgns from your network at your OpenCPN