The exploits I've seen are in DDRJake's Ryukyu campaign - starting the game with a 3* leader by repeating missions to boost tradition. This kind of thing is basically cheating as far as I'm concerned. Not sure what you mean by ' "use overseas coring discount" via vassal block' - sounds like it's probably complicated but 'as intended'. To be honest, I think we all know what's exploitative, but we might disagree over what's acceptable.
Ryukyu is always my main circus exhibition of exploits. If you want some straight-up weak starts play without exploitation (at least until I'm fairly powerful) then I would recommend Nagaur, Mogadishu, Mongolia or Navarra.
In the end though, I enjoy finding and displaying exploits in this game. For my exploits magnum opus, I recommend Minghals