Mangel,
I know you are asking about the Navigation system, and I cannot answer that question.
But more importantly, from what I'm hearing, and what Tink is reconfirming, is that
without the Navi there is no instantaneous mpg display.
You may not realize it now, and current hybrid owners will tell you, that without some kind of instantaneous display of mpg, it will be very hard to reach the full mpg potential of your car.
Without the Navi all you'll end up with is a trip cumulative average of your mpg. While the cumulative average mpg is helpful, it still does not help you with fine tuning your mpg technique.
Now you could add an aftermarket gauge for the instantaneous mpg for under $150. See this thread
ScanGauge on Honda Accord Hybrid
Again, I realize this doesn't answer the Navigation add-on question, but thought I should point out this important fact.