You can run the wires along the lower frame tubes and up to the steering head on the frame attaching the wires periodically with cable ties. It's sort of the "ugly" way to do it even if you tuck the wires behind the tube basically out of sight.
It isn't actually hard to run the wires under the seat and tank and you might be able to do it without completely removing the tank. By just loosening the tank you might be able to lift it enough to "fish" a thin wire, like guitar string size, under it and then pull your wiring through. Even removing the tank isn't very hard and if you decide to do that I'll let you in on the tricks and what you need to do.