I don't think this is a Construct 2 issue: as you have basically identified yourself it appears to be a problem with either Node-webkit, OS X or HFS+. Construct 2 uses unicode support throughout, as well as the standard unicode-aware Windows APIs when writing and copying files, so I'm not sure what we could change there. Sometimes people raise similar issues where misconfigured web servers use some different characterset and mangle UTF-8 filenames, then return 404 not found. I'm not sure to what extent it's Construct 2's responsibility to solve that.
Sticking to ASCII names sucks for non-English people, but is a pragmatic way to avoid these problems. You say the zip trick works too, but note that it could increase the loading time of the application (since it must first decompress the zip).