Overtype mode in Terminal.app
I don't have access to my Mac right now, but on my Linux machine I'm set up to use zsh as my default shell.  It looks like you can do this on Mac too.  Once you're in zsh, you can easily use Vim keybindings  (I guess it may also work in Bash?) by running set -o vi.
With Vim bindings active, you can press ESC to go to Command Mode, then use any normal Vim keys including R to switch to Overtype mode.
PS: To go back to Emacs bindings use set -o emacs.