Any chance that anyone has the actual wiring diagram for that PIAA harness? If you can suss out what goes where, it should be fairly easy to sort out what needs to be connected to what to make a light bar work (I'm looking at adding a light bar too, so this is useful to me as well).
If I had to guess, the handlebar switch controls a low-amp circuit from the fuse block to a relay. The relay also draws from the fuse block and controls the current to the actual lights (that's the usual way of doing this in a plug-and-play installation). The "proper" way to do this would be to have the circuit for the lights connect directly to the battery, but that takes extra wire and means disassembling more of the bike.
So, presumably, there's a free high-amp terminal somewhere on the fuse block. The whole harness plugs into that terminal, and has two circuit loops: one low-amp with the bar switch that just operates the relay, and one high amp for the lights which should be wired in parallel to each other (so if one dies you'll still have one that works). It's 50/50 whether they have two high-amp switches in the relay for a completely redundant path for each light, or one single path through the relay which splits off to the lights "downstream". Two separate paths is more reliable but means twice as much wire.
You should be able to just seal off one of the light connectors, and connect to the other one to power your bar; you can leave everything else in place. The trick will be to find the matching plug to wire to your lightbar's pigtail so you can quick-connect it. If nothing else, you have a second wiring loop to connect the bar to in case you develop a wiring fault in the loop your bar is currently connected to.