While not in use the current draw by the converter should be minimal (your clock probably draws more current) so I don't see a problem plugging it directly to the battery through a fuse (for protection, in case of short things can blow/catch fire if not behind a fuse). Only thing is if you forget your phone or something else for awhile you could kill the battery.
The R3 doesn't seem to have any accessory wiring to which you could easily tap into. If you want to have the charger turn on on keyturn you would need to use a relay switched by something that gets turned on by keyturn. Might be a bit complicated for such a simple accessory. Wish I could help you more but I don't have my bike yet, otherwise I would tinker around and make some instructions.