I recently did a similar overview of terminals since I needed to pass all keypresses to the terminal, I ended up with something like this in xterm, but I hear it works in urxvt as well:

     .Xdefaults-localhost
     *VT100.Translations: #override \
              	Shift<Key>Return: string("shift return")\n\
              	Ctrl<Key>Return: string("ctrl return")\n\
              	Alt<Key>Return: string("alt return")\n\
              	Super<Key>Return: string("super return")

Here are the escape sequences used by Kitty for sending modifiers to the shell from the terminal: https://github.com/kovidgoyal/kitty/blob/master/protocol-ext... (thanks aumerle)