If you want to go Flashless on Mac and Safari, it's possible to use Firefox as a fallback, too, not just Chrome. While Firefox does load Plugins from
/Library/Internet Plug-Ins and
~/Library/Internet Plug-Ins, it looks in other places too. I just tested and it seems to work fine from
~/Library/Application Support/Firefox/Profiles/<profile name>/plugins and I suspect
/Applications/Firefox.app/Contents/MacOS/plugins would work too.
So you can copy
flashplayer.xpt to one of the Firefox specific folders and launch Firefox from Safari's Developer menu. Chrome starts up faster, though.