1. Insert Reason as a rewire chanel
2. Add an instrument in Reason
3. Add a virtual instrument track to Mixcraft
4. When you play notes on the VI track it also plays the currently selected instrument in Reason.
My suggestion is a way to only send notes to a Reason device if you select it as an output on a Virtual Instrument track.
It would also be great if Mixcraft allowed us access to the 64 outputs that Rewire & Reason provide, much like how the current multi-out VST's work.