There is a spare plug on the RHS near the horn mounting, you need to take the side fairings off to see it, it is quite concealed along the frame towards the front of the bike. From memory it is the lap trigger canbus socket. You would need a plug from bmw to attach to your sat nav power supply/cable or get them to do it for you, then plug it in and mount the nav. That way you have a switched power solution that turns off approx 10 seconds after the bike.
Wiring directly to the battery is fine (in some ways its is better because it avoids the canbus), but you need to have an inline fuse, and if you accidentally leave the nav in and on it will drain your battery.