Twitter client for OSX that posts via proxy?
You can use Spotflux. It runs on system level which means you can use Twitter for OS X and other apps without having to install additional plugins in browsers.
There are also other paid alternatives like TunnelBear or NetShade. I personally use NetShade because it doesn’t require Java like Spotflux.
If I understand your question you can use Hola as a free VPN and simulate you're in an other (selectable) country. Works great, installable as plugin in firefox and google chrome.