That would be incredibly cool but the Forerunners weren't designed to be navigation tools, they only use the GPS to track your position and based on their sampling rate determine your approximate speed and then, using the Training Centre software translates that into a map of your ride. To be able to provide navigation assistance you'd need a bigger display, built-in mapping etc. and I don't think even their navigation units could accommodate what you're looking for as they only map roadways.

Would you really want to be referring to a GPS navigation unit while riding your bike? You're going to miss a lot of the scenery and, I suspect, it would be a major distraction......