Sounds like a neat idea to reduce the noise if that is indeed a problem. I can't see where it would harm anything if the insert can't/doesn't shift around at all and somehow block the bore.
I'd agree w/eightbore on this one, and if the sounds of gunfire bother someone,,then the shooting sports are not what they should pursue. The Winchester 42 is certainly what I'd consider a 'fine firearm' and has been discussed and pictured here many times in the past. I think many others would agree.