After seeing a tweet from @GotNoSugarBaby
I thought "that seems doable", and it turns out it is!
As long as you're using iTerm2, you can bind hotkeys to "Send Text", which allows "\n" for newlines.
Simply open the preferences window (cmd+,) and switch to the "Keys" pane. Add a new global shortcut key as shown in the screenshot above (The original version of this post used "!!\n", but using the escape sequence for up as in the updated version also works in non-bash scenarios).
Now you can "Reload" your last command by bashing "cmd+r" - web-dev browser synergy here we come!