FWIW I have several bikes equipped with Zumos, and I have always hard wired directly to the battery. I have measured the Zumo current draw in the charging state, and it is about 0.5 Amps. The Garmin supplied power cable is fused, so you need not install an additional fuse block. IMO, this is the simplest way to go and avoids any issues, small as they might be, involving the Canbus. A couple of crimped lugs after routing the cable to the battery and you are done.