No amount of to-and-fro will make a function that does not exist suddenly spring into existence. If you want a date format that differs from the locale implied one then you need to code for it. There's no mechanism that can intercept all argument-free calls to QDateTime::toString() and QLocale equivalent. Even if there were, the default item delegate for item views formats the date explicitly in QLocale::ShortFormat so you would still have to replace the delegates relying on default format.

If you think such an ability is globally important then by all means code it, submit for inclusion in Qt, and fight for it. Since you will be proposing breaking compatibility with all existing Qt code you will need to wait a while to get it in.