Apparently with the Zumo you can save a route to your SD card as a GPX file. Create routes using ITN Converter which uses Google Maps.
Whilst this isn't using Google Maps via the browser, but using it's API, it makes planning routes on a PC without Mapsource installed easy. ITN Converter is small (260KB) and will run off a USB memory stick. If you have a USB memory stick style SD card reader (peanuts from eBay) then you can have ITN Converter on your Zumo SD card and simply take it out, put into reader, plug into PC, run ITN Converter saving the routes back to the card. Ideal for route planning at Internet cafes whilst away from home etc.
Bear in mind that the settings on your Zumo and any mapping differences between Google Maps and City Nav on your Zumo might give slightly different routes.