Yang pointed me to a program that emulates a Garmin.. then use whatever program you use to get the data into the Garmin and this will output a .RTE file with the route information the program uploaded. This .RTE file has all the waypoints on the route and their latitude longitude location, so it would not be hard to write a program to get this into a format the parse_maponus script would understand.
Problem though: The route waypoint names really suck since Garmin has a limit of like 3 characters on waypoint names. Another problem: Depending on what program you use to download the information to your Garmin, the route shape points may not be available (route shape points are points that define the road shapes). Either that or the emulator does not display them.
I'm working on a system to get around these two restrictions with Delorme Street Atlas since it lets you export the route waypoint names and the route shape points to a text file. Haven't had much time to work on it though with my classes starting again though..