You can do it in Mapsource (Preferences, Routing, Advanced) but AFAIK not on the GPS.
I think the best bet would be to draw 2 short sections of track to go the way you think will be best in these areas, and load these onto the GPS along with the route. Go into the Track (or Course) menu and colour them bright and (important) set them to 'show on map'.
Then on the day just let the routing do its own thing but follow your tracks to get round the problem.