Sorry, I can't say that I have experience, but I had thought of doing this.
However, I had a couple of different concerns:
Chipping of the powder coat from the leaf springs flexing/rubbing, and the effect of the heat required for powder coating. (I don't know if they get hot enough to affect the properties of the spring steel or not).
I'm sure someone with more experience with powder coating springs will chime in here - Maybe it works out OK. It sure seems like powder coat would look good!
-Tom