ok, but that's even worse than what I thought you were asking me to do... Rewire MIDI is all handled behind the curtain, it does not show as an available midi device in your OS, so no, you can't select "rewire MIDI" in the drop down box in your EMI (and I did check it to be sure even though I knew what I would find)
Even if it did have that option, sonar would only be able to compensate for the latency that it's already expecting from Reason, It would have no way of determining latency from a third program not part of the rewire protocol. Rewire isn't magic, it all has to obey physics. at the end of the day you are using a VST host to play a VST through Reason, which is just weird. presumably you were going to use this setup to save all your midi data in Reason, and then open it later in reason standalone with all the latency taken care of?
what is a LOT easier is just using MIDI out normally, rendering to audio when you are ready, and manually snapping the audio to the timing in the sequencer in Reason. it's not really that big a deal. and if you just want to deal with the MIDI data and not render just yet, you can adjust the timing (not perfectly though) using the slide function in the ReGroove mixer. that's much simpler and easier than setting up a convoluted mousetrap using rewire and another app, even if it would be possible which it isn't.
you are thinking outside the box though, that's good.