Despite earlier reports of problems, I've fitted an additional horn to my 1200 without any need for electrical jiggery pokery. This demonstrates that the circuit supplying the horn isn't THAT fussy about the current it supplies.
Remember that the horn itself isn't connected directly to the infamous 2 wire bus - there's a "black box" controlled by the bus that turns the horn circuit on and off when ordered to. The black box also turns the circuit off if the current draw goes outside certain limits (high or low) - which is what people have been expecting would cause problems. But, at least on my bike, the upper limit on the current draw for the horn circuit isn't reached when driving two horns.
I haven't experimented with the lights yet, but I'd expect the usual method of wiring a relay in parallel with the headlamp bulb (dip or full beam depending on your requirements), and then running power direct from the battery for the addition lights, via the relay, to work.