I made the following Squat Rx #18 about concentric and zercher squats primarily because I had requests for instruction on the two exercises. To be honest, I've never done either of them consistently, although I've enjoyed doing them as supplemental lifts here and there. I didn't address the "arm pain issue" with zerchers because I don't think there is really much of a solution to it - it's an uncomfortable lift and besides padding the bar (and increasing the circumference of the bar, possibly adding new problems), there's not getting around it.
If you have thoughts, I hope you'll post them; here or at YouTube, or shoot me an email or PM. I look forward to hearing from you.