The 520 does do on the fly navigation.
You can kind of do courses and live segments at the same time: in each course options, you can choose to always display it. If you do that and don't select "ride course", you can see the course outline on the map and live segments work as intended.
The "Guide Text" in maps is pretty useless to me. What I would like is the option to show the next course point in that same place at the top of maps. Now you would have something REALLY nice.