'Legger,
The direct answer to your question is that you can anneal it. That takes the spring temper out of it, and moves the yield point down to where it would be easy to tweak. Just heat it red hot with a torch and let it slowly air cool.
That said, I wouldn't do it that way. If you ask the question a bit differently, you get a whole 'nuther set of answers that provide you with better choices.
1. Use the piano wire, but build the airplane with tweaking in mind. To do this, you will likely need to stay below 1/8" wire or things just get too difficult. Next, you need to keep the joiner short enough that it will yield before your surfaces run out of hinge travel. For example, you will likely be unsuccessful with a 3/32 x 6" joiner. Finally, the flap or elevator has to be built in a way that is suitable for tweaking. Just drilling a hole in 6 lb/ft^3 balsa isn't going to do it. It has to be reinforced somehow to keep from smashing the wood and loosening everything up. Plywood, FRP and other materials used for sockets--often referred to as "Lucky boxes" do this. As a bare minimum, soaking the hole with thin CyA and then cleaning out the hole helps. Next, use some hard wood blocks clamped over both surfaces to avoid point loads (spread out the load) and you will be able to tweak a music wire joiner.
2. Make your own horns using A2 tool steel. This is air-hardening, so it is pretty easy to braze and then heat-treat and retemper if you feel the need. Derek provided a very detailed set of instructions about how he builds horns using these materials.
3. If you want micro-adjustability and like making metal parts then try one of the split horns. I think there have been a couple posted here. The trick is to make it accessible inside the fuselage, small enough so that it isn't clunky, and reliable enough that you won't regret not just picking option (1) above.
Hope this helps,
Dave
PS--I will try to put the other thread references into this post so people can backtrack....
PPS--I recently tweaked the flaps on my regifted OPP "yellow 'pants". I was pretty confident that it had a store-bought horn in it. I could not tell exactly how the flaps were built or even the entry point of the joiner. I could tell from flexing them that it was less than 1/8" (torque vs. windup), so I got out the bending blocks and went to it. It went fine, with no damage to the plane. And it flies much straighter! If I had felt the wood in the flaps start to give, I would have had to splice in new flap-ends with proper reinforcement and repaint the flaps. Of course, I could of used a large trim tab, but that kind of offended me....