Using the rear signal source is interesting. They may even be able to trigger small relays if not able to take the additional loading alone.
That is how I installed my lights, from the rear harness post ECU. The kit runs a primary 12v and taps into the turn signal wires for signal only. The controller does the rest. There are actually two 12v (light green, I think) wires that supply the rear signals with power on the bike itself. My 12v source power comes from it. I have some photos of it in my Adventures in Valkyrie Wiring post.