If your computer has a bi-directional midi interface, you could hook up your fantom to send and receive midi from your computer. I don't own a fantom, but if the fantom has a usb port that can be used as a midi interface, you can use that as well.
Then you can use Cubase to send midi down to your fantom. To do this, create a midi channel in Cubase, set the midi-in of that channel to the midi port that the fantom is connected to, and set the midi-out of the channel to the port the fantom is connected to (usually they are the same)
You would still need to have the audio connected into an audio channel in Cubase, but you could set that channel to monitor the audio, and drive your fantom from Cubase's midi out. When you record, you can record the midi and during playback, the midi will drive the fantom just as it drives softsynths. Then you have the flexibility to edit the midi in Cubase if you wish.
The fantom should have a setting to turn off local control to elimate echos from both your keyboard and the midi channel when recording or monitoring.
This is what I do when I have my Juno-D hooked up into my computer, and it works great. I am using it as a combination midi-controller and sound module.
Of couse, this approach would not work with Reason alone since it doesn't support midi-out or audio-in.
Hope this helps.
cycloptic