An easy solution for the usb, if you don't want to muck with oem wiring, is to install a switch in serial w the USB and just go right to the battery. It's not as elegant of course, but it's pretty easy and if you mount a lighted switch or lighted usb port it would be easy to see that it's on so you remember to turn it off. This is what I did on my k75c back in the day and it worked well. Probably want to fuse that circuit if you do that.
Otherwise, you could connect it to the heated grips accessory wire....pretty sure all k bikes have this connector, even if they don't have the grips. Might be taped to your frame(left side iirc) with a plastic cap on the end. For some reason im.remembering the cap is blue, but might be brown, idk.