I've always owned Zumo models for my bikes. I remember trying so many times to solve the same problem you have, powering Zumo from USB. When I purchased my new 1250GS back in January, the problem came to me again, as new 1250GS doesn't have 12v plug by the handlebars. I know Garmin sells especial cable to power your Zumo by USB but besides cable being very expensive, you lose resistance to moisture and water ingress because the rubber cover of USB plug has to be open. Then, depending on your model of Zumo, you may not be able to put the unit into its holding bracket because of the usb cable connected.
You cannot use just any USB cable because the unit will think it's connected to computer and enter file transfer mode. That is why you need Garmin special USB cable. There are some hacks if you use standard USB cable, from cutting one of the pins to plugging/unplugging USB cable while unit is booting up to trick it out.
For me, it was too much hassle so I purchased Wunderlich bracket to install DIN plug on the left side of TFT, also purchased DIN plug for panel mount and DIN connector that I soldered on the end of Zumo's bracket cable (instead of hard wiring it onto the bike).
