@joaoevangelista thanks for the suggestion - sent me off down the path of serving static files and came across this thread Static files not found where the OP was referencing files which didn’t actually exist.
My issue was that in app.html.eex I was using MYApp.prod.app.js yet the actual file being produced was MyApp.prod.app.js. OSX is more forgiving than CentOS when it comes to such things.
@NobbZ thanks for your help as well 
Sorry for wasting time! Hopefully this helps someone else in the same scenario, though - even if the filenames seem correct, double, no triple check!
